declare compatibility constant as canonical gref for primitive projections

Author: Tragicus

elpi/coq-lib.elpi

@@ -510,6 +510,7 @@ coq.term->gref (global GR) GR :- !.
 coq.term->gref (pglobal GR _) GR :- !.
 coq.term->gref (app [Hd|_]) GR :- !, coq.term->gref Hd GR.
 coq.term->gref (let _ _ T x\x) GR :- !, coq.term->gref T GR.
+coq.term->gref (primitive (proj Proj _)) GR :- !, primitive-projection? Proj GR.
 :name "term->gref:fail"
 coq.term->gref Term _ :-
   fatal-error-w-data "term->gref: input has no global reference" Term.