- bidirectional, i.e. if the source(s) can be determined from the target
- unidirectional, i.e., if multiple source(s) reference the same target(s)

It is mandatory that any concrete AllocationEntrys inherit either IBidirectional or IUnidirectional.

TODO: Correctly handle IUnidirectional allocation entries in the getSource/TargetElement[s] eOperations of AllocationTable
refs 2950