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

def _get_substitute_element(head, elt, ps):
    '''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
    '''
    if not isinstance(head, ElementDeclaration):
        return None

    return ElementDeclaration.getSubstitutionElement(head, elt, ps)