code: def p_union(self, p):
          '''union : UNION IDENTIFIER '{' field_seq '}' annotations'''
          p[0] = ast.Union(
              name=p[2], fields=p[4], annotations=p[6], lineno=p.lineno(2)
          f p_union(self, p):
          '''union : UNION IDENTIFIER '{' field_seq '}' annotations'''
          p[0] = ast.Union(
              name=p[2], fields=p[4], annotations=p[6], lineno=p.lineno(2)
          )
docs: union : UNION IDENTIFIER '{' field_seq '}' annotations