def _required_child(parent, tag):
    """
    Add child element with *tag* to *parent* if it doesn't already exist.
    """
    if _child(parent, tag) is None:
        parent.append(_Element(tag))
def _required_child ( parent , tag ) : if _child ( parent , tag ) is None : parent . append ( _Element ( tag ) )
Add child element with *tag* to *parent* if it doesn't already exist.