The invariance is needed so that only a single template is needed if a material is used on structures that are defined in opposite directions for source/target. That way the open end of the bag can be specified to always align with the open end of the tube. At the moment there is no way to specify that invariant without carefully checking which direction things are specified in (and switching the bag type manually if they change).
Current
<==<vs>==>Desired
<==>vs>==<The invariance is needed so that only a single template is needed if a material is used on structures that are defined in opposite directions for source/target. That way the open end of the bag can be specified to always align with the open end of the tube. At the moment there is no way to specify that invariant without carefully checking which direction things are specified in (and switching the bag type manually if they change).