---input---
Code fence:

```
foo
```

Code fence with language:

```python
import this
```

Code fence with unknown language:

```invalid-lexer
foo
```

---tokens---
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' '           Text
'fence:'      Text
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' '           Text
'fence'       Text
' '           Text
'with'        Text
' '           Text
'language:'   Text
'\n'          Text.Whitespace

'\n```'       Literal.String.Backtick
'python'      Literal.String.Backtick
'\n'          Text

'import'      Keyword.Namespace
' '           Text
'this'        Name.Namespace
'\n'          Text.Whitespace

'```\n'       Literal.String.Backtick

'\n'          Text.Whitespace

'Code'        Text
' '           Text
'fence'       Text
' '           Text
'with'        Text
' '           Text
'unknown'     Text
' '           Text
'language:'   Text
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'\n'          Text.Whitespace

'```invalid-lexer' Text
'\n'          Text.Whitespace

'foo'         Text
'\n'          Text.Whitespace

'```'         Text
'\n'          Text.Whitespace
