code: function addGuard(guard, expr) {
        let safeExpr = null;
        if (!isWellFormed(expr)) {
          expr = '{/*Malformed Expression*/}';
        }
        needsRuntime = true;
        safeExpr = ` rt_${ unpredictableSuffix }.${ guard }(${ expr }) `;
        return safeExpr;
      }
docs: Decorate expression text with a call to a guard function.