code: function() {
      var node = this.relational();
      while (this.accept('in')) {
        this.operand = true;
        if (!node) this.error('illegal unary "in", missing left-hand operand');
        node = new nodes.BinOp('in', node, this.relational());
        this.operand = false;
      }
      return node;
    }
docs: relational ('in' relational)*