code: public SingleFieldBuilder<MType, BType, IType> mergeFrom(
        MType value) {
      if (builder == null && message == message.getDefaultInstanceForType()) {
        message = value;
      } else {
        getBuilder().mergeFrom(value);
      }
      onChanged();
      return this;
    }
docs: Merges the field from another field.
  
  @param value the value to merge from
  @return the builder