def add_lic_xref(self, doc, ref):
if self.has_extr_lic(doc):<EOL><INDENT>self.extr_lic(doc).add_xref(ref)<EOL>return True<EOL><DEDENT>else:<EOL><INDENT>raise OrderError('<STR_LIT>')<EOL><DEDENT>
Adds a license cross reference.
        Raises OrderError if no License ID defined.