It throws an exception:
```python
    v.variables.append(DetailVariable(name='case', function='@case_id'))
  File "/home/simon/.virtualenvs/commcarehq/local/lib/python2.7/site-packages/eulxml/xmlmap/fields.py", line 740, in append
    self[len(self)] = x
  File "/home/simon/.virtualenvs/commcarehq/local/lib/python2.7/site-packages/eulxml/xmlmap/fields.py", line 705, in __setitem__
    match = _create_xml_node(self.xast, self.node, self.context, insert_index)
  File "/home/simon/.virtualenvs/commcarehq/local/lib/python2.7/site-packages/eulxml/xmlmap/fields.py", line 262, in _create_xml_node
    return _create_xml_node(xast.right, left_node, context)
  File "/home/simon/.virtualenvs/commcarehq/local/lib/python2.7/site-packages/eulxml/xmlmap/fields.py", line 241, in _create_xml_node
    new_node = _create_child_node(node, context, xast, insert_index)
  File "/home/simon/.virtualenvs/commcarehq/local/lib/python2.7/site-packages/eulxml/xmlmap/fields.py", line 278, in _create_child_node
    new_node = E(step.node_test.name)
  File "/home/simon/.virtualenvs/commcarehq/local/lib/python2.7/site-packages/lxml/builder.py", line 208, in __call__
    elem = self._makeelement(tag, nsmap=self._nsmap)
  File "lxml.etree.pyx", line 2808, in lxml.etree.Element (src/lxml/lxml.etree.c:61442)
  File "apihelpers.pxi", line 93, in lxml.etree._makeElement (src/lxml/lxml.etree.c:13533)
  File "apihelpers.pxi", line 1517, in lxml.etree._tagValidOrRaise (src/lxml/lxml.etree.c:26739)
ValueError: Invalid tag name u'*'
```