A sentinel transformer object
Squash TreeTransform's beloning to same sublist to a single TreeTransformer Each TreeTransform gets own period, whereas a combined TreeTransformer gets period equal to union of periods of it's TreeTransforms
Use the following phases in the order they are given.
Use the following phases in the order they are given. The list should never contain NoPhase. if squashing is enabled, phases in same subgroup will be squashed to single phase.