1 files changed, 18 insertions, 0 deletions
diff --git a/src/Util/Sigma/Associativity.v b/src/Util/Sigma/Associativity.v
new file mode 100644
index 000000000..fbebf2584
--- /dev/null
+++ b/src/Util/Sigma/Associativity.v
@@ -0,0 +1,18 @@
+(** * Reassociation of [sig] *)
+Definition sig_sig_assoc {A} {P : A -> Prop} {Q}
+  : { a : A | P a /\ Q a } -> { ap : { a : A | P a } | Q (proj1_sig ap) }
+  := fun apq => exist _ (exist _ (proj1_sig apq) (proj1 (proj2_sig apq))) (proj2 (proj2_sig apq)).
+Ltac sig_sig_assoc :=
+  lazymatch goal with
+  | [ |- { a : ?A | ?P } ]
+    => let P'' := fresh a in
+       let P' := fresh P'' in
+       let term := constr:(fun a : A => match P with
+                                        | P' => ltac:(let v := (eval cbv [P'] in P') in
+                                                      lazymatch eval pattern (proj1_sig a) in v with
+                                                      | ?P _ => exact P
+                                                      end)
+                                        end) in
+       let Q := lazymatch (eval cbv beta in term) with fun _ => ?term => term end in
+       apply (@sig_sig_assoc _ _ Q)
+  end.