I can migrate an activity instance under another activity instance that is not in the ancestor path to the root activity instance.

Example:
See horizontal_migration.png. The first process definition is mapped to the second, A is mapped to A, B is mapped to B, subprocess is mapped to subprocess.

Current behavior:
See horizontal_migration2.png. There are two instances of the subprocess.

Expected behavior:
See horizontal_migration3.png. There should be a way to determine that the instance of A becomes a child of the existing subprocess instance. Note that this may require to select amongst a larger set of suitable subprocess instances.

Note: this behavior should be optional in addition to the current behavior.