-- Why delete? Consider
-- let x = a*b in (x, \x -> x+3)
-- We add [x |-> a*b] to the substitution, but we must
- -- *delete* it from the substitution when going inside
+ -- _delete_ it from the substitution when going inside
-- the (\x -> ...)!
modifyInScope :: SimplEnv -> CoreBndr -> CoreBndr -> SimplEnv