(* the delift function takes in input a metavariable index, a local_context
* and a term t, and substitutes every subterm t' of t with its position
* (searched up-to unification) in
(* the delift function takes in input a metavariable index, a local_context
* and a term t, and substitutes every subterm t' of t with its position
* (searched up-to unification) in