+
+(* Replaces in "where" every term in "what" with the corresponding
+ term in "with_what". The terms in "what" ARE lifted when binders are
+ crossed. The terms in "with_what" ARE lifted when binders are crossed.
+ Every free variable in "where" IS NOT lifted by nnn.
+ Thus "replace_lifting_csc 1 ~with_what:[Rel 1; ... ; Rel 1]" is the
+ inverse of subst up to the fact that free variables in "where" are NOT
+ lifted. *)