DenotTransformers that change the signature of their denotation's info must go after erasure. The reason is that denotations are permanently referred to by TermRefs which contain a signature. If the signature of a symbol would change, all refs to it would become outdated - they could not be dereferenced in the new phase.
After erasure, signature changing denot-transformers are OK because erasure will make sure that only term refs with fixed SymDenotations survive beyond it. This is possible because:
- splitter has run, so every ident or select refers to a unique symbol
- after erasure, asSeenFrom is the identity, so every reference has a plain SymDenotation, as opposed to a UniqueRefDenotation.