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fix tautological lemma bits.skipn_app #258

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Merged
andres-erbsen merged 1 commit into from
Apr 30, 2026
+10 −3
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fix tautological lemma bits.skipn_app #258

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7 changes: 7 additions & 0 deletions doc/changelog/01-changed/258-bits-skipn-app.rst
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@@ -0,0 +1,7 @@
- in `theories/Zmod/Bits.v`

+ Fixed lemma `bits.skipn_app` to actually describe the interaction between
the titular functions. Developments relying on the previous tautological
statement can resore it by adding that version of the lemma to the codebase
(`#258 <https://github.com/coq/stdlib/pull/258>`_,
by Andres Erbsen).
6 changes: 3 additions & 3 deletions theories/Zmod/Bits.v
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Expand Up @@ -415,11 +415,11 @@ Proof.
Qed.

Lemma skipn_app [n m] a b (Hn : 0 <= n) (Hm : 0 <= m) :
skipn n (@app n m a b) = of_Z _ (to_Z (skipn n (@app n m a b))).
skipn n (@app n m a b) = of_Z _ (to_Z b).
Proof.
case (skipn_app_ex a b) as [?->]; trivial.
apply to_Z_inj; rewrite to_Z_eq_rect, to_Z_of_Z.
rewrite Z.add_simpl_l, mod_to_Z; trivial.
apply to_Z_inj. rewrite to_Z_eq_rect, to_Z_of_Z.
pose proof unsigned_range b. rewrite Z.mod_small; lia.
Qed.

Lemma app_assoc_dep [n m l] (a : bits n) (b : bits m) (c : bits l)
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