<?xml version="1.0" encoding="UTF-8"?>
<xsd:schema xmlns:xsd="http://www.w3.org/2001/XMLSchema"
            xmlns="http://www.ivoa.net/xml/VODML/v1.0"
            targetNamespace="http://www.ivoa.net/xml/VODML/v1.0"
            attributeFormDefault="unqualified" elementFormDefault="unqualified">

  <xsd:annotation>
    <xsd:documentation>
    
      This schema defines the "vo-dml meta model", a modeling language used to define the concepts
      and structures that describe data models. It is proposed as the common language for
      defining data models in the IVOA.
TBD continue based on VO-DML specification document.
     </xsd:documentation>
    <xsd:documentation>
    Look at SVN versions 2129 and earlier for more extensive model.
    </xsd:documentation>
  </xsd:annotation>

  <!-- +++++++++++++++++++ Begin of 'Identifier section' +++++++++++++++++++ -->
  <xsd:simpleType name="VODMLID" >
    <xsd:annotation>
      <xsd:documentation>
          Type representing the way referable elements are identified uniquely in VO-DML.
      </xsd:documentation>
    </xsd:annotation>
      <xsd:restriction base="xsd:string">
          <xsd:annotation>
        <xsd:documentation>
        A restriction on the valid strings that make this a VO-DML Identifier.
        Requirements on the format are: 1) use in XML document 2) use as fragment in URI
        </xsd:documentation>
      </xsd:annotation>
          <xsd:pattern value="[\w\._-]+"/>
      </xsd:restriction>
  </xsd:simpleType>

  <xsd:simpleType name="ModelName">
    <xsd:annotation>
      <xsd:documentation>
        Type used to restrict valid values for prefixes.
        TBD We could use an xsd:NCName for this.
      </xsd:documentation>
    </xsd:annotation>
    <xsd:restriction base="xsd:string"> <!-- xsd:NCName ? -->
      <xsd:pattern value="[a-zA-Z][\w_-]*">
        <xsd:annotation>
          <xsd:documentation>
            A model name MUST NOT contain a semicolon.
          </xsd:documentation>
        </xsd:annotation>
      </xsd:pattern>
    </xsd:restriction>
  </xsd:simpleType>

  <xsd:simpleType name="VODMLREF">
    <xsd:annotation>
      <xsd:documentation>
        Class representing the way ReferencableElements can be referenced in VO-DML.
        It must be possible to refer to elements in other, imported data models as well as in the current model.
        Hence the VODMLREF must identify both model and element.
        The element is identified by the VO-DML ID in the model, the model is identified using a
          prefix that MUST correspond to the vodml-id element of the current or an imported model.
          Note, references to element sin the current model MUST also have a prefix, there is no default model!
        TBD We could use an xsd:QName where VODMLREF is used, but that may have somewhat more sever syntax constraints than desired.
      </xsd:documentation>
    </xsd:annotation>
      <xsd:restriction base="xsd:string"> <!--  xsd:QName ?  -->
      <xsd:annotation>
        <xsd:documentation>
          A restriction on the valid strings that make this a valid reference to a referencable element.
            Consists of a prefix that should follow the ModelPrefix restriction
            and a identifier that should follow the VODMLID restriction,
          separated from each other by a colon.
        </xsd:documentation>
        <xsd:documentation>
          TODO define the restriction pattern properly.
          MUST be [Prefix ':' VODMLID].
        </xsd:documentation>
      </xsd:annotation>
          <xsd:pattern value="[\w_-]+:[\w-/\._]+"/>
      </xsd:restriction>
  </xsd:simpleType>

  <xsd:simpleType name="VODMLName">
    <xsd:annotation>
      <xsd:documentation>
        Type used to restrict valid values for prefixes.
        TBD We could use an xsd:NCName for this.
      </xsd:documentation>
    </xsd:annotation>
    <xsd:restriction base="xsd:string"> <!-- xsd:NCName ? -->
      <xsd:pattern value="[a-zA-Z_][\w_]*">
        <xsd:annotation>
          <xsd:documentation>
            A model name MUST NOT contain a semicolon.
          </xsd:documentation>
        </xsd:annotation>
      </xsd:pattern>
    </xsd:restriction>
  </xsd:simpleType>


  <xsd:complexType name="ReferableElement" abstract="true">
    <xsd:annotation>
      <xsd:documentation>
        This is the base type for all types whose elements can be explicitly referenced.
        To this end it has a 'vodml-id' element of type VODMLID that allows explicit and unique identification of 
        these elements within the context of the model.
        Generally these are also elements that can be
        represented explicitly in alternative serialisations of
        a data model, such as a VOTable or a relational model.
        These should use the value of the utype element to "point into a data model" and identify a
        model element. VO-DML itself also has needs of pointing to other elements, sometimes in another model.
        The VODMLREF type is used for such references, which will always be named 'vodml-ref'.
      </xsd:documentation>
    </xsd:annotation>
    <xsd:sequence>
      <xsd:element name="vodml-id" type="VODMLID" minOccurs="1">
        <xsd:annotation>
          <xsd:documentation>
              Identifier for its containing element.
              Extracted as a separate type so that we can easily adapt to a different identifier design.
          </xsd:documentation>
        </xsd:annotation>
      </xsd:element>
      <xsd:element name="name" type="VODMLName" minOccurs="1" >
        <xsd:annotation>
          <xsd:documentation>
            The name of the model element. 
            MUST NOT be an empty string.
          </xsd:documentation>
        </xsd:annotation>
      </xsd:element>
      <xsd:element name="description" type="xsd:string" minOccurs="0">
        <xsd:annotation>
          <xsd:documentation>
            Human readable description of the model element.
          </xsd:documentation>
        </xsd:annotation>
      </xsd:element>
    </xsd:sequence>

    <xsd:attribute name="id" type="xsd:string" use="optional">
      <xsd:annotation>
        <xsd:documentation>
          A referencable element may be given an @id attribute to reflect an identifier
          defined in some source document form which a VO-DML model may have been derived.
        </xsd:documentation>
      </xsd:annotation>
    </xsd:attribute>
  </xsd:complexType>


  <xsd:complexType name="ElementRef">
    <xsd:annotation>
      <xsd:documentation>
        This type represents how to reference a ReferencableElement.
        It can serve as base class to those types that explicitly reference another type, such as relations and roles.
        It provides for a uniform way to represent the reference to
        the target element using the 'utyperef' element.
        An important design choice is that we wish to allow references to elements in remote models.
        For that reasons we can not use an ID/IDREF or key/keyref pattern.
        Instead we define various constraints on
        this type and its usage in various contexts using
        the Schematron file in vo-dml.sch.xml.
      </xsd:documentation>
    </xsd:annotation>
    <xsd:sequence>
      <xsd:element name="vodml-ref" type="VODMLREF">
        <xsd:annotation>
          <xsd:documentation>
            The element identifying the referenced target element.
            See the documentation for the VODMLREF type.
          </xsd:documentation>
        </xsd:annotation>
      </xsd:element>
    </xsd:sequence>
  </xsd:complexType>


  <!-- +++++++++++++++++++ End of 'ReferencableElement section' +++++++++++++++++++ -->

  <!-- +++++++++++++++++++ Begin of Model elements section +++++++++++++++++++ -->
    <xsd:complexType name="Model">
        <xsd:annotation>
            <xsd:documentation>
                Represents a complete data model and is the type of the (single) declared root element for
                VO-DML/XML representation documents.
            </xsd:documentation>
    </xsd:annotation>
        <xsd:sequence>
            <xsd:element name="name" type="ModelName" minOccurs="1" maxOccurs="1">
                <xsd:annotation>
                    <xsd:documentation>
                        Short name of the model.
                        NOTE this name MUST be used as prefix in any utype reference to elements in this model.
                    </xsd:documentation>
        </xsd:annotation>
            </xsd:element>
            <xsd:element name="description" type="xsd:string" minOccurs="0" maxOccurs="1">
                <xsd:annotation>
                    <xsd:documentation>
                        The description of the model.
                    </xsd:documentation>
                </xsd:annotation>
            </xsd:element>
            <xsd:element name="title" type="xsd:string" minOccurs="1" maxOccurs="1">
                <xsd:annotation>
          <xsd:documentation>
              The title of the model by which it is officially known.
          </xsd:documentation>
                </xsd:annotation>
            </xsd:element>
            <xsd:element name="author" type="xsd:string" minOccurs="0" maxOccurs="unbounded">
                <xsd:annotation>
                    <xsd:documentation>
                    List of authors of the model, only defined by name so far.
            TBD could be expanded with email, affiliation and so on.
          </xsd:documentation>
        </xsd:annotation>
      </xsd:element>
      <xsd:element name="version" type="xsd:string" minOccurs="1">
        <xsd:annotation>
          <xsd:documentation>
            Label giving the version of the model.
          </xsd:documentation>
        </xsd:annotation>
      </xsd:element>
      <xsd:element name="previousVersion" type="xsd:anyURI" minOccurs="0">
        <xsd:annotation>
          <xsd:documentation>
            URI identifying a VO-DML model that is the version from which the current version of model is derived.
            TBD could be an IVO Identifier once models get properly registered?
          </xsd:documentation>
        </xsd:annotation>
      </xsd:element>
      <xsd:element name="lastModified" type="xsd:dateTime">
        <xsd:annotation>
          <xsd:documentation>
            Timestamp when the last change to the current model was made.
          </xsd:documentation>
        </xsd:annotation>
      </xsd:element>
      <xsd:element name="import" type="ModelImport" minOccurs="0" maxOccurs="unbounded">
        <xsd:annotation>
          <xsd:documentation>
            An 'import' element indicates a dependency on an external, predefined VO-DML data model.
            Types from that model may be used for referencing, extension and assignment to
            attributes.
            Types from the external model MUST NOT be used for
            composition relationships.
            'identification' relations to elements from that model may be used to indicate some kind of
            equivalence between
            elements in the current model and the external elements.
          </xsd:documentation>
          <xsd:documentation>
            TBD We might require that every data model MUST include a version of the IVOA data model
            to gain access to the standard
            primitive types and some other types.
            We may require that that standard model should be included *completely*,
            i.e. including all its type definitions explicitly.
            This would be similar to treating it as a UML Profile, rather than an import.
            This would mean that the most common type assignments for attributes
            can be checked within the model and not require
            importing the remote model during validation.
          </xsd:documentation>
        </xsd:annotation>
      </xsd:element>
            <xsd:element name="primitiveType" type="PrimitiveType" minOccurs="0" maxOccurs="unbounded">
                <xsd:annotation>
                    <xsd:documentation>
                        Collection of PrimitiveType definitions directly under the model, i.e. not contained in a
                        Package.
                    </xsd:documentation>
                </xsd:annotation>
            </xsd:element>
            <xsd:element name="enumeration" type="Enumeration" minOccurs="0" maxOccurs="unbounded">
                <xsd:annotation>
                    <xsd:documentation>
                        Collection of Enumeration definitions directly under the model, i.e. not contained in a Package.
                    </xsd:documentation>
                </xsd:annotation>
            </xsd:element>
            <xsd:element name="dataType" type="DataType" minOccurs="0" maxOccurs="unbounded">
                <xsd:annotation>
                    <xsd:documentation>
                        Collection of DataType definitions directly under the model, i.e. not contained in a Package.
                    </xsd:documentation>
                </xsd:annotation>
            </xsd:element>
            <xsd:element name="objectType" type="ObjectType" minOccurs="0" maxOccurs="unbounded">
                <xsd:annotation>
                    <xsd:documentation>
                        Collection of ObjectType definitions directly under the model, i.e. not contained in a Package.
                    </xsd:documentation>
                </xsd:annotation>
            </xsd:element>
            <xsd:element name="package" type="Package" minOccurs="0" maxOccurs="unbounded">
                <xsd:annotation>
                    <xsd:documentation>
                        The collection of packages which can contain further detailed name spacing to
                        the type definitions in the model.
                    </xsd:documentation>
                </xsd:annotation>
            </xsd:element>
        </xsd:sequence>
    </xsd:complexType>


    <xsd:complexType name="ModelImport">
    <xsd:annotation>
      <xsd:documentation>
        A "proxy" for an external model that is being used by the current model. 
        Defines the url where the VO-DML representation of that model can be retrieved, and
        replicates its name that MUST be used when making references to
        elements in that model using a VODMLREF element.
      </xsd:documentation>
    </xsd:annotation>
    <xsd:sequence>
        <xsd:element name="name" type="ModelName" minOccurs="1">
            <xsd:annotation>
                <xsd:documentation>
                    Name by which imported model is used in the current model and its documentation.
                    This name MUST be the same as the 'name' of the model definition in that remote document.
                    For all utypes pointing to elements in the imported model MUST use this name as prefix.
                </xsd:documentation>
        </xsd:annotation>
      </xsd:element>
        <xsd:element name="version" type="xsd:string" minOccurs="0">
            <xsd:annotation>
                <xsd:documentation>
                    Version of the imported model.
                </xsd:documentation>
            </xsd:annotation>
        </xsd:element>
      <xsd:element name="url" type="xsd:anyURI" minOccurs="1">
        <xsd:annotation>
          <xsd:documentation>
            URL from which the VO-DML model document can be downloaded.
            Note, could likely be done through a registry once ivoId is known.
            TBD SHOULD this be a generic URI, or can we insits on URL?
          </xsd:documentation>
        </xsd:annotation>
      </xsd:element>
      <xsd:element name="documentationURL" type="xsd:anyURI" minOccurs="1">
        <xsd:annotation>
          <xsd:documentation>
            URL where a documentation HTML file for the remote model can be downloaded.
            This SHOULD be a document that contains anchors for each element thta has as name attribute the vodml-id of that element.
            I.e. it is assumed that the
            vodml-id-s of the imported types can be added onto this documentationURL
            (should end with a #?) so that a direct link to the documentation for a referenced data model element can be found.
          </xsd:documentation>
        </xsd:annotation>
      </xsd:element>
    </xsd:sequence>
  </xsd:complexType>


    <xsd:complexType name="Package">
        <xsd:annotation>
            <xsd:documentation>
                A Package is a container for type definitions and possible (child-)packages.
                Names of types only need to be unique within their container (model or package),
                hence a package provides further name-spacing for type definitions.
                When
                deriving physical representations of a model, packages may be mapped to containers in the target
                meta-model.
                For example in mapping to XSD they may give rise to separate documents with type definitions and their
                own targetNamespace. When generating
                Java classes they may be used to define seprate packages for
                the classes derived form the types.
            </xsd:documentation>
        </xsd:annotation>
        <xsd:complexContent>
            <xsd:extension base="ReferableElement">
                <xsd:annotation>
                    <xsd:documentation>
                        Name of the package is constrained in that
                        there can only be one package with a given name
                        in the container in which the package is defined, i.e. the model or a parent package.
                        TBD we may wish to extend this rule to all children
                        defined in a container, including types.
                    </xsd:documentation>
                </xsd:annotation>
                <xsd:sequence>
                    <xsd:element name="primitiveType" type="PrimitiveType" minOccurs="0" maxOccurs="unbounded">
                        <xsd:annotation>
                            <xsd:documentation>
                                Collection of PrimitiveType-s defined in this package.
                            </xsd:documentation>
                        </xsd:annotation>
                    </xsd:element>
                    <xsd:element name="enumeration" type="Enumeration" minOccurs="0" maxOccurs="unbounded">
                        <xsd:annotation>
                            <xsd:documentation>
                                Collection of Enumeration-s defined in this package.
                            </xsd:documentation>
                        </xsd:annotation>
                    </xsd:element>
                    <xsd:element name="dataType" type="DataType" minOccurs="0" maxOccurs="unbounded">
                        <xsd:annotation>
                            <xsd:documentation>
                                Collection of DataType-s defined in this package.
                            </xsd:documentation>
                        </xsd:annotation>
                    </xsd:element>
                    <xsd:element name="objectType" type="ObjectType" minOccurs="0" maxOccurs="unbounded">
                        <xsd:annotation>
                            <xsd:documentation>
                                Collection of ObjectType-s defined in this package.
                            </xsd:documentation>
                        </xsd:annotation>
                    </xsd:element>
                    <xsd:element name="package" type="Package" minOccurs="0" maxOccurs="unbounded">
                        <xsd:annotation>
                            <xsd:documentation>
                                Collection of child Package-s defined in this package.
                            </xsd:documentation>
                        </xsd:annotation>
                    </xsd:element>
                </xsd:sequence>
            </xsd:extension>
        </xsd:complexContent>

    </xsd:complexType>


    <xsd:complexType name="Type" abstract="true">
    <xsd:annotation>
      <xsd:documentation>
        Base class of all type definition elements.
        All Type-s extend ReferenceableElement, i.e. they are referencable.
        Adds name, description, inheritance and indication of abstractness to ReferencableElement.
      </xsd:documentation>
      <xsd:documentation>
        Name of the type. Must be unique in the collection of all types in a given container
        (i.e. model or package)
      </xsd:documentation>
    </xsd:annotation>
    <xsd:complexContent>
      <xsd:extension base="ReferableElement">
        <xsd:sequence>
          <xsd:element name="extends" type="ElementRef" minOccurs="0">
            <xsd:annotation>
              <xsd:documentation>
                Reference to a type (called the base-type) that is extended by the current type (called the subtype).
                This implements the typical is-a inheritance relationship, similar to the extends relations in XSD and Java,
                the
                generaliation in UML, or the subclassOf relation in RDF. Note, VO-DML does not support multiple inheritance.
                Instances of a subtype are automatic instances of a base type.
                Polymorphism is assumed: When a role (see below) defines a base type
                as its datatype, instances of any subtype
                can be uased as value of the role.
                Roles defined on a base type are inherited by the subtypes.
                Relations inherited from a basetype can be 'subsetted', which is similar to overriding their definition.
                See the definiton of this property on the Relation type.
              </xsd:documentation>
            </xsd:annotation>
          </xsd:element>
            <xsd:element name="constraint" type="Constraint" minOccurs="0" maxOccurs="unbounded">
                <xsd:annotation>
                    <xsd:documentation>
                        Constraints defining valid instances of the type.
                        May be an AttributeConstraint or an expression in some language.
                    </xsd:documentation>
                </xsd:annotation>
            </xsd:element>

        </xsd:sequence>
        <xsd:attribute name="abstract" type="xsd:boolean" default="false" use="optional" />
      </xsd:extension>
    </xsd:complexContent>
  </xsd:complexType>

  <xsd:complexType name="ObjectType">
    <xsd:annotation>
      <xsd:documentation>
        TBD use description form VO-DML specification document. to ...
      </xsd:documentation>
    </xsd:annotation>
    <xsd:complexContent>
      <xsd:extension base="Type">
        <xsd:sequence>
          <xsd:element name="attribute" type="Attribute" minOccurs="0" maxOccurs="unbounded">
            <xsd:annotation>
              <xsd:documentation>
                TODO
              </xsd:documentation>
            </xsd:annotation>
          </xsd:element>
            <xsd:element name="collection" type="Composition" minOccurs="0" maxOccurs="unbounded">
            <xsd:annotation>
              <xsd:documentation>
                TODO
              </xsd:documentation>
            </xsd:annotation>
          </xsd:element>
          <xsd:element name="reference" type="Reference" minOccurs="0" maxOccurs="unbounded">
            <xsd:annotation>
              <xsd:documentation>
                TODO
              </xsd:documentation>
            </xsd:annotation>
          </xsd:element>
        </xsd:sequence>
      </xsd:extension>
    </xsd:complexContent>
  </xsd:complexType>


  <xsd:complexType name="ValueType" abstract="true">
    <xsd:annotation>
      <xsd:documentation>
        Base class of all valaue types, i.e. those types identified by their value, rather than a separate explicit identifier.
        These are the types that can be assigned to Attribute-s.
      </xsd:documentation>
    </xsd:annotation>
    <xsd:complexContent>
      <xsd:extension base="Type">
      </xsd:extension>
    </xsd:complexContent>
  </xsd:complexType>


  <xsd:complexType name="PrimitiveType">
    <xsd:annotation>
      <xsd:documentation>
        Atomic/simple type. Defined by a single value. Generally a built in type from the IVOA profile model,
        or a subclass of one of those types.
      </xsd:documentation>
    </xsd:annotation>
    <xsd:complexContent>
      <xsd:extension base="ValueType">
      </xsd:extension>
    </xsd:complexContent>
  </xsd:complexType>

  <xsd:complexType name="DataType">
    <xsd:complexContent>
      <xsd:extension base="ValueType">
        <xsd:sequence>
          <xsd:element name="attribute" type="Attribute" minOccurs="0" maxOccurs="unbounded">
            <xsd:annotation>
              <xsd:documentation>
                TODO
              </xsd:documentation>
            </xsd:annotation>
          </xsd:element>
          <xsd:element name="reference" type="Reference" minOccurs="0" maxOccurs="unbounded">
            <xsd:annotation>
              <xsd:documentation>
                TODO
              </xsd:documentation>
            </xsd:annotation>
          </xsd:element>
        </xsd:sequence>
      </xsd:extension>
    </xsd:complexContent>
  </xsd:complexType>

  <xsd:complexType name="Enumeration">
    <xsd:annotation>
      <xsd:documentation>
        A primitive type with a limited, discrete set of values.
        May explicitly extend a PrimitiveType. Its values must be compatible with that type then.
        TBD Should define what it
        might mean for an enumeraiton to extend another enumeration.
        Should it restrict the possible values further? Or should it add to the values? Or ...?
      </xsd:documentation>
    </xsd:annotation>
    <xsd:complexContent>
      <xsd:extension base="PrimitiveType">
        <xsd:sequence>
          <xsd:element name="literal" type="EnumLiteral" maxOccurs="unbounded">
            <xsd:annotation>
              <xsd:documentation>
                TODO
              </xsd:documentation>
            </xsd:annotation>
          </xsd:element>
        </xsd:sequence>
      </xsd:extension>
    </xsd:complexContent>
  </xsd:complexType>

  <xsd:complexType name="EnumLiteral">
    <xsd:complexContent>
      <xsd:extension base="ReferableElement">
        <xsd:sequence>
         </xsd:sequence>
      </xsd:extension>
    </xsd:complexContent>
  </xsd:complexType>

  <xsd:complexType name="Role" abstract="true">
    <xsd:annotation>
      <xsd:documentation>
        A Role represents the "role a Type plays in the definition of another Type".
        Generally, instances of structured types contain instances of other types, organised according to some
        predesigned pattern consisting basically of
        name-value pairs.
        The names refer to the particular role to which the values are assigned.
        These values must have the type corresponding to the role, implemented below using the datatype element.
        The values may be multiple-valued.
        Three different types
        of roles are supported in VO-DML: Attribute, COllection and Reference.
        Their characteristics are defined below.
      </xsd:documentation>
      <xsd:documentation>
        Role extends ReferencableElement.
        The 'name' element that is inherited from that type must be unique in the collection of roles
        defined on the parent type.
        This uniqueness must extend over the roles available on the type by
        inheritance.
      </xsd:documentation>
    </xsd:annotation>

    <xsd:complexContent>
      <xsd:extension base="ReferableElement">
        <xsd:sequence>
          <xsd:element name="datatype" type="ElementRef">
            <xsd:annotation>
              <xsd:documentation>
                Reference to the type that plays the role represented by this Role.
              </xsd:documentation>
            </xsd:annotation>
          </xsd:element>
          <xsd:element name="multiplicity" type="Multiplicity">
            <xsd:annotation>
              <xsd:documentation>
                The multiplicity of the role (also called cardinality) indicates whether it must have a
                value or may be without value, or possibly how many values are allowed.
              </xsd:documentation>
            </xsd:annotation>
          </xsd:element>
          <xsd:element name="subsets" type="ElementRef" minOccurs="0">
            <xsd:annotation>
              <xsd:documentation>
                Represents the UML subsetted property. Indicates that a particular relation refines the definition
                of another relation. ONly a relation inherited form a base class can
                be subsetted. Typical usage is
                that the base class has a
                relation to a class A, and the subclass refines this to indicating that
                the relation should be to a subclass of A.

                The value should identify the subsetted property.
                TBD IF we are going to use utype-s to refer to elements also inside this
                document, we should use an
                appropriate keyref
              </xsd:documentation>
              <xsd:documentation>
                TBD this is a somewhat abstract, but very useful modeling concept.
                It implements a very common modeling design pattern.
                It exists in UML2.
              </xsd:documentation>
            </xsd:annotation>
          </xsd:element>
        </xsd:sequence>
      </xsd:extension>
    </xsd:complexContent>
  </xsd:complexType>

  <xsd:complexType name="Attribute">
    <xsd:annotation>
      <xsd:documentation>
        An Attribute is a Role where the target datatype is a ValueType.
        It represent "simple" properties of its container type, which can be an ObjectType or a DataType.
      </xsd:documentation>
      <xsd:documentation>
        Must refer to a ValueType.
      </xsd:documentation>
    </xsd:annotation>
    <xsd:complexContent>
      <xsd:extension base="Role">
        <xsd:sequence>
          <xsd:element name="semanticconcept" type="SemanticConcept" minOccurs="0">
            <xsd:annotation>
              <xsd:documentation>
                It is possible to assign a SEmanticConcept to an attribute definition.
                This means that the values of the attribute have to comply with the definition of the 
                SemanticConcept.
                This can be done in two manners. Either the SemanticConcept
                gives a link to a semantic vocabulary, in which case the value must be a
                concept defined in that vocabulary.
                Or it defines a topConcept, in which case the value must be a  concept that is explicitly
                declared to be (narrower than)
                that concept, or a concept that is narrower than that concept.
                FOr details on the interpretation see the VO-DML document.
              </xsd:documentation>
            </xsd:annotation>
          </xsd:element>
        </xsd:sequence>
      </xsd:extension>
    </xsd:complexContent>
  </xsd:complexType>



  <xsd:complexType name="SemanticConcept">
    <xsd:annotation>
      <xsd:documentation>
        Type used to indicate on attributes that they take values representing a concept defined in
        an identified semantic vocabulary (SKOS or RDFS), and/or restricted by being narrower/more specific than an
        identified "top" concept.
      </xsd:documentation>
    </xsd:annotation>
    <xsd:sequence>
      <xsd:element name="topConcept" type="xsd:anyURI" minOccurs="0">
        <xsd:annotation>
          <xsd:documentation>
            A URI identifiying a semantic concept that corresponds to the concept in the model.
            Values of a corresponding attributes must be URI-s identifiying objects that are narrower
            than the identified concept. This attribute may be null as
            certain vocabularies may not have a
          </xsd:documentation>
        </xsd:annotation>
      </xsd:element>
      <xsd:element name="vocabularyURI" type="xsd:anyURI" minOccurs="0" maxOccurs="unbounded">
        <xsd:annotation>
          <xsd:documentation>
            If no topConcept is defined, one or more explicit vocabularies can be provided from which the
            value must be obtained.
          </xsd:documentation>
        </xsd:annotation>
      </xsd:element>
    </xsd:sequence>
  </xsd:complexType>

  <xsd:complexType name="Relation" abstract="true">
    <xsd:annotation>
      <xsd:documentation>
        A relation is a Role where the target datatype is an ObjectType.
      </xsd:documentation>
    </xsd:annotation>
    <xsd:complexContent>
      <xsd:extension base="Role">
        <xsd:sequence>
        </xsd:sequence>
      </xsd:extension>
    </xsd:complexContent>
  </xsd:complexType>

  <xsd:complexType name="Reference">
    <xsd:annotation>
      <xsd:documentation>
        A Reference is a Relation that indicates a kind of "usage" relationship
        between the target ObjectType and the owner of the reference, the "referrer".
        The referrer can be an ObjectType (typically) but also a DataType.
        The relation is
        looser than the composition/collection relationship, acting like a
        semantically meaningful pointer rather than indicating a component of the referrer.
        Consequently, in general many referrers can point at the same target instance,
        and ObjectType-s can
        be the target in different reference definitions.
        The lifecycle of the target is not bound to that of the referrer.
        Often the target instance is used to provide a context for the definition of
        the referrer. For example a coordinate frame may be
        referenced to provide context to coordinate values.
        TBD more needed ...?
      </xsd:documentation>
    </xsd:annotation>
    <xsd:complexContent>
      <xsd:extension base="Relation">
        <xsd:annotation>
          <xsd:documentation>
            TBD Should have multiplicity 0..1 or 1?
          </xsd:documentation>
        </xsd:annotation>
      </xsd:extension>
    </xsd:complexContent>
  </xsd:complexType>

    <xsd:complexType name="Composition">
        <xsd:annotation>
      <xsd:documentation>
          This type implements a composition relation between the parent and child ObjectTypes.
          Its instances are ONLY used to set the "collection" field on an ObjectType.
          It is a rule that an object type can only be the target of a single collection definition.
          A subclass can be assigned a target to a collection if a
        baseclass is already assigned such a target, but only if the collection is explicitly 'subsetted'.
        A collection is assumed to be a set, i.e.
        a given object (as identified by its identifier!) cannot occur
        multiple times in the collection.
        The collection
        may be ordered, whichi implies that the order in whichi objects have been added
        to
        the collection is to be preserved. As clients can always do an explicit sort on any of the child objects' attributes,
        it seems not necessary to add functionality for
        declaring a collection is
        sorted on one or more attributes.
        Through the uniqueInCollection constraint that can be assigned to attributes, a collection can impose the
        constraint that different objects in the collection
        must have distinct values of the
        attribute to which that constraint is assigned.
        It would be better probably to add the capability to assign such constraints to this collection type.
        This would
        also give more flexibility in for example creating explicit (named) keys, or defining
        multi-attribute uniqueness constraints.
      </xsd:documentation>
    </xsd:annotation>
    <xsd:complexContent>
      <xsd:extension base="Relation">
        <xsd:sequence>
          <xsd:element name="isOrdered" type="xsd:boolean" default="false" minOccurs="0">
            <xsd:annotation>
              <xsd:documentation>
                If true, this collection preserves the ordering of object insertions.
              </xsd:documentation>
            </xsd:annotation>
          </xsd:element>
        </xsd:sequence>
      </xsd:extension>
    </xsd:complexContent>
  </xsd:complexType>


  <xsd:complexType name="Multiplicity">
    <xsd:annotation>
      <xsd:documentation>
        Also called "Cardinality". Indicates how many instances of a datatype can/must be associated to a given role.
        Unless
        Follows model in XSD, i.e. with explicit lower bound and upper bound on number of instances.
        maxOccurs must be gte minOccurs, unless it is negative, in which case it corresponds to unbounded.
      </xsd:documentation>
    </xsd:annotation>
    <xsd:sequence>
      <xsd:choice>
      <xsd:element name="minOccurs" type="xsd:nonNegativeInteger" default="1">
        <xsd:annotation>
          <xsd:documentation>
          Lower bound on number of instances/values.
          </xsd:documentation>
        </xsd:annotation>
      </xsd:element>
			</xsd:choice>
      <xsd:element name="maxOccurs" type="xsd:int" default="1">
        <xsd:annotation>
          <xsd:documentation>
          When negative, unbounded.
          </xsd:documentation>
        </xsd:annotation>
      </xsd:element>
    </xsd:sequence>
  </xsd:complexType>


    <xsd:complexType name="Constraint">
        <xsd:annotation>
      <xsd:documentation>
      Constraint represents rules that instances of Type-s must obey to be valid.  
      A generic Constraint only has a 'description' element 
      which describes the constraint in English.
      In future versions of the language extra elements may be added that give a more formal
      definition of the constraint. In particular we may add expressions in a language
      such as OCL or subset thereof tuned to VO-DML.
      In terms of OCL, VO-DML COnstraint-s are invariants of a Type.
      </xsd:documentation>
    </xsd:annotation>
    <xsd:sequence>
      <xsd:element name="description" type="xsd:string" minOccurs="0">
        <xsd:annotation>
          <xsd:documentation>
          A natural language description of the constraint.
          </xsd:documentation>
        </xsd:annotation>
      </xsd:element>
    </xsd:sequence>
    </xsd:complexType>
    
    
    <xsd:complexType name="SubsettedRole">
        <xsd:annotation>
      <xsd:documentation>
      A special type of constraint that represents the common pattern where a subtype.
      would like to restrict certain aspects of a definition of a  roles defined on a super type.
      The constraint explicitly defines the datatype of the identified role. This new datatype MUST
      be a subtype of the declared datatype of the role that is being constrained. Similarly
      a new, more restrictive semantic concept  may be defined.
      </xsd:documentation>
    </xsd:annotation>
    <xsd:complexContent>
    <xsd:extension base="Constraint">
      <xsd:sequence>
        <xsd:element name="role" type="ElementRef"> 
          <xsd:annotation>
            <xsd:documentation>
            VODMLREF identifying the constrained Role.
            This role MUST be available to the type containing this constraint.
            </xsd:documentation>
          </xsd:annotation>
        </xsd:element>
        <xsd:element name="datatype" type="ElementRef" minOccurs="0">
          <xsd:annotation>
            <xsd:documentation>
            Pointer to datatype that the constrained Role must take.
            This datatype MUST be a sub-type of the declared datatype of the constrained Role.
            </xsd:documentation>
          </xsd:annotation>
        </xsd:element>
        <xsd:element name="semanticconcept" type="SemanticConcept" minOccurs="0">
          <xsd:annotation>
            <xsd:documentation>
						Maybe the super type has not defined a semantic concept for the Role, but 
						the subtype needs that. This attribute allows this assignment. But alse when 
						the Role on the super-type already has a semanticconcept with a topConcept 
						defined on it, the subtype may restrict the values to a narrower concept than 
						that assigned to it on the super-type.             
				    </xsd:documentation>
          </xsd:annotation>
        </xsd:element>
      </xsd:sequence>
    </xsd:extension>
    </xsd:complexContent>
    
    </xsd:complexType>

  <!-- Begin of element declaration(s) -->
  <xsd:element name="model" type="Model">
    <xsd:annotation>
      <xsd:documentation>
        Every VO-DML/XML document must start with a 'model' element, no other root elements are supported by this spec.
      </xsd:documentation>
    </xsd:annotation>
  </xsd:element>


</xsd:schema>
