code: function _not(expr) {
        var state = this.bt.current_state();
        try { var r = expr.call(this) } 
        catch (f) {
          this.bt.catch_mismatch(f);
          this.bt.restore(state);
          return true
        }
        this.bt.mismatch("Negative lookahead didn't match got " + r);
      }
docs: negative lookahead ~rule