public void registerHiddenBuiltIn(Class<? extends GenericUDF> functionClass) {
    lock.lock();
    try {
      if (!isNative) {
        throw new RuntimeException("Builtin is not for this registry");
      }
      builtIns.add(functionClass);
    } finally {
      lock.unlock();
    }
  }
Registers the UDF class as a built-in function; used for dynamically created UDFs, like
GenericUDFOP*Minus/Plus.
ql/src/java/org/apache/hadoop/hive/ql/exec/Registry.java