- tci_co :: Either -- co
- TcTyVar -- - a wanted equation, with a hole, to be
- -- filled with a witness for the equality;
- -- for equation arising from deferring
- -- unification, 'ty1' is the actual and
- -- 'ty2' the expected type
- Coercion, -- - a given equation, with a coercion
- -- witnessing the equality;
- -- a coercion that originates from a
- -- signature or a GADT is a CoVar, but
- -- after normalisation of coercions, they
- -- can be arbitrary Coercions involving
- -- constructors and pseudo-constructors
- -- like sym and trans.