@Override
 public Optional<EyeLocationData> fill(
     DataHolder dataHolder, EyeLocationData manipulator, MergeFunction overlap) {
   if (supports(dataHolder)) {
     final EyeLocationData merged = overlap.merge(manipulator.copy(), from(dataHolder).get());
     return Optional.of(manipulator.set(Keys.EYE_HEIGHT, merged.eyeHeight().get()));
   }
   return Optional.absent();
 }
 @Override
 public DataTransactionResult set(
     DataHolder dataHolder, EyeLocationData manipulator, MergeFunction function) {
   if (!supports(dataHolder)) {
     return DataTransactionBuilder.failResult(manipulator.getValues());
   }
   final ImmutableValue<Double> newValue = manipulator.eyeHeight().asImmutable();
   final EyeLocationData oldManipulator = from(dataHolder).get();
   final ImmutableValue<Double> oldValue = oldManipulator.eyeHeight().asImmutable();
   final EyeLocationData newData = function.merge(oldManipulator, manipulator);
   ((IMixinEntity) dataHolder).setEyeHeight(newData.eyeHeight().get());
   return DataTransactionBuilder.successReplaceResult(newValue, oldValue);
 }