Adopt str4d's suggestion.

Signed-off-by: Daira-Emma Hopwood <daira@jacaranda.org>

CHANGELOG.md

@@ -7,8 +7,8 @@ and this library adheres to Rust's notion of
 
 ## [Unreleased]
 ### Added
-- `zcash_spec::PrfExpand::{REGISTERED_ZIP32_CHILD, with_tag}` (for tagged ZIP 32
-  child derivation).
+- `zcash_spec::PrfExpand::REGISTERED_ZIP32_CHILD` (for tagged ZIP 32 child
+  derivation).
 
 ## [0.1.2] - 2024-10-01
 ### Added

src/prf_expand.rs

@@ -96,6 +96,7 @@ impl PrfExpand<([u8; 32], [u8; 4])> {
     pub const SPROUT_ZIP32_CHILD: Self = Self::new(0x80);
     pub const ORCHARD_ZIP32_CHILD: Self = Self::new(0x81);
     pub const ARBITRARY_ZIP32_CHILD: Self = Self::new(0xAB);
+    pub const REGISTERED_ZIP32_CHILD: Self = Self::new(0xAC);
 }
 impl PrfExpand<([u8; 32], [u8; 32])> {
     pub const ORCHARD_DK_OVK: Self = Self::new(0x82);
@@ -109,22 +110,11 @@ impl PrfExpand<([u8; 96], [u8; 32], [u8; 4])> {
 }
 with_inputs!(a, A, b, B, c, C);
 
-impl PrfExpand<([u8; 32], [u8; 4])> {
+impl PrfExpand<([u8; 32], [u8; 4], [u8; 1], &[u8])> {
     pub const REGISTERED_ZIP32_CHILD: Self = Self::new(0xAC);
 
-    /// Expands the given secret key in this domain, with additional `lead`
-    /// and `tag` inputs.
-    pub fn with_tag(
-        self,
-        c_par: &[u8],
-        sk_par: &[u8; 32],
-        i: &[u8; 4],
-        lead: Option<u8>,
-        tag: &[u8],
-    ) -> [u8; 64] {
-        match lead {
-            None => self.apply(c_par, &[sk_par, i, tag]),
-            Some(b) => self.apply(c_par, &[sk_par, i, &[b], tag]),
-        }
+    /// Expands the given secret key in this domain.
+    pub fn with(self, sk: &[u8], a: &[u8; 32], b: &[u8; 4], c: &[u8; 1], d: &[u8]) -> [u8; 64] {
+        self.apply(sk, &[a, b, c, d])
     }
 }