code: def p_example_field(self, p):
          
          if p[3] is NullToken:
              p[0] = AstExampleField(
                  self.path, p.lineno(1), p.lexpos(1), p[1], None)
          else:
              p[0] = AstExampleField(
                  self.path, p.lineno(1), p.lexpos(1), p[1], p[3])
docs: example_field : ID EQ primitive NL
                           | ID EQ ex_list NL
                           | ID EQ ex_map NL