+ Cic.metasenv -> Cic.context -> Cic.term -> Cic.term ->
+ substitution * Cic.metasenv
+
+(* fo_unif_subst metasenv subst context t1 t2 *)
+(* unifies [t1] and [t2] in a context [context] *)
+(* and with [subst] as the current substitution *)
+(* (i.e. unifies ([subst] [t1]) and *)
+(* ([subst] [t2]) in a context *)
+(* ([subst] [context]) using the metasenv *)
+(* ([subst] [metasenv]) *)
+(* Only the metavariables declared in [metasenv] *)
+(* can be used in [t1] and [t2]. *)
+(* [subst] and the substitution returned are not *)
+(* unwinded. *)
+(*CSC: fare un tipo unione Unwinded o ToUnwind e fare gestire la
+ cosa all'apply_subst!!!*)
+val fo_unif_subst :
+ substitution -> Cic.context -> Cic.metasenv -> Cic.term -> Cic.term ->
+ substitution * Cic.metasenv
+
+(* unwind_subst metasenv subst *)
+(* unwinds [subst] w.r.t. itself. *)
+(* It can restrict some metavariable in the [metasenv] *)
+val unwind_subst : Cic.metasenv -> substitution -> substitution * Cic.metasenv