diff --git a/schemas/sturdy.prs b/schemas/sturdy.prs
index 6d36735..0481cda 100644
--- a/schemas/sturdy.prs
+++ b/schemas/sturdy.prs
@@ -4,8 +4,9 @@ embeddedType EntityRef.Cap .
 ; The sequence of Caveats is run RIGHT-TO-LEFT.
 ; That is, the newest Caveats are at the right.
 ;
-; Let f = HMAC-BLAKE2s, e = canonical machine-oriented serialization of some preserves value,
-; and k = the original secret key for the ref.
+; Let f(k,d) = HMAC-BLAKE2s-256(k,d)[0..16),
+;     e = canonical machine-oriented serialization of some preserves value, and
+;     k = the original secret key for the ref.
 ;
 ; The `sig` is then f(f(f(f(k, e(oid)), ...), Caveat), ...).
 ;