+(* 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 local_context (which is the Rel moved to the canonical context).
+ * Typically, the list of optional terms is the explicit
+ * substitution that is applied to a metavariable occurrence and the result of
+ * the delift function is a term the implicit variable can be substituted with
+ * to make the term [t] unifiable with the metavariable occurrence. In general,
+ * the problem is undecidable if we consider equivalence in place of alpha
+ * convertibility. Our implementation, though, is even weaker than alpha
+ * convertibility, since it replace the term [tk] if and only if [tk] is a Rel
+ * (missing all the other cases). Does this matter in practice?
+ * The metavariable index is the index of the metavariable that must not occur
+ * in the term (for occur check).
+ *)