def _get_substitute_element(head, elt, ps):
if not isinstance(head, ElementDeclaration):<EOL><INDENT>return None<EOL><DEDENT>return ElementDeclaration.getSubstitutionElement(head, elt, ps)<EOL>
if elt matches a member of the head substitutionGroup, return 
    the GED typecode.

    head -- ElementDeclaration typecode, 
    elt -- the DOM element being parsed
    ps -- ParsedSoap Instance