Steven Watt suggests using a "homogeneous" search for
dispatching to method functions, i.e., if 

   x in X0 < X1 < ...
   y in Y0 < Y1 < ...

and f(x,y) is entered, then look for methods in this order:

    f(X0,Y0)
    f(X0,Y1) f(X1,Y0)
    f(X0,Y2) f(X1,Y1) f(X2,Y0)
    ...

Is there an efficient way to do it?
