get_peer

        High-level api: Find all children under the node with the tag.

        Parameters
        ----------

        tag : `str`
            A tag in `{namespace}tagname` notaion.

        node : `Element`
            A node to be looked at.

        Returns
        -------

        Element or None
            None if not found. An Element object when found.

def get_peer(tag, node):
        '''get_peer

        High-level api: Find all children under the node with the tag.

        Parameters
        ----------

        tag : `str`
            A tag in `{namespace}tagname` notaion.

        node : `Element`
            A node to be looked at.

        Returns
        -------

        Element or None
            None if not found. An Element object when found.
        '''

        peers = node.findall(tag)
        if len(peers) < 1:
            return None
        elif len(peers) > 1:
            raise ModelError("not unique tag '{}'".format(tag))
        else:
            return peers[0]