+(*DEBUGGING ONLY:
+prerr_endline (Printf.sprintf
+"Pullback problem\nterm1: %s\nterm2: %s\ncar1: %s\ncar2: %s\nlast_tl1: %s
+last_tl2: %s\nhead1_c: %s\nhead2_c: %s\n"
+(CicMetaSubst.ppterm_in_context ~metasenv subst (C.Appl l1) context)
+(CicMetaSubst.ppterm_in_context ~metasenv subst (C.Appl l2) context)
+(CoercDb.string_of_carr car1)
+(CoercDb.string_of_carr car2)
+(CicMetaSubst.ppterm_in_context ~metasenv subst last_tl1 context)
+(CicMetaSubst.ppterm_in_context ~metasenv subst last_tl2 context)
+(CoercDb.string_of_carr head1_c)
+(CoercDb.string_of_carr head2_c)
+);
+*)