diff --git a/theories/proofmode/coq_tactics.v b/theories/proofmode/coq_tactics.v
index 9e971ae75af93bdc0b1857c9fbd711614f62b407..86966e8c59a41e9f91045e974b428f64d925d31c 100644
--- a/theories/proofmode/coq_tactics.v
+++ b/theories/proofmode/coq_tactics.v
@@ -524,6 +524,8 @@ Proof.
 Qed.
 
 (** * Pure *)
+(* This relies on the invariant that [FromPure false] implies
+   [FromPure true] *)
 Lemma tac_pure_intro Δ Q φ af :
   env_spatial_is_nil Δ = af → FromPure af Q φ → φ → envs_entails Δ Q.
 Proof.