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diff --git a/src/Util/Tactics/UnfoldArg.v b/src/Util/Tactics/UnfoldArg.v
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+++ b/src/Util/Tactics/UnfoldArg.v
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+Require Import Crypto.Util.Tactics.ChangeInAll.
+
+(** fully reduces the first argument to [t], wherever it appears *)
+Ltac unfold_first_arg t :=
+  repeat match goal with
+         | [ H : context[t ?x] |- _ ] => progress change_with_compute_in_all x
+         | [ |- context[t ?x] ] => progress change_with_compute_in_all x
+         end.
+
+(* use uconstr so we can have underscores *)
+Tactic Notation "unfold_first_arg" uconstr(t) := unfold_first_arg t.