message: existence filters
diff: @@ -94,6 +94,13 @@ module.exports = declareInjections({
  }
  }).filter(Boolean));
  }
  + if (value.exists != null){
  + if (String(value.exists) === "false") {
  + return [...expression, "is null"];
  + } else {
  + return [...expression, "is not null"];
  + }
  + }
  throw new Error("Unimplemented field value");
  }
  