Map `stmt` to a NETMOD-specific attribute.

        The name of the attribute is the same as the 'keyword' of
        `stmt`.

def nma_attribute(self, stmt, p_elem, pset=None):
        """Map `stmt` to a NETMOD-specific attribute.

        The name of the attribute is the same as the 'keyword' of
        `stmt`.
        """
        att = "nma:" + stmt.keyword
        if att not in p_elem.attr:
            p_elem.attr[att] = stmt.arg