A compiler pass is given by a WellFormedProgramAlgebra from its source
language features to its target language features. They support a "small
footprint" style of specification where only the features needed but be
declared. The typeclass mechanism will lift the compiler pass into any pair of
source/target languages that are supersets of the wanted features.
For instance, RemoveUnaryIfs contains a program algebra that compiles away
unary if constructs into binary if constructs, with a unit in the else branch.
This is currently done in an untyped fashion.
Ideally, but not implemented yet, it would come with a WellFormedProofAlgebra
guaranteeing that the transformation is semantics-preserving w.r.t. an
evaluation semantics for the language features.