Document the consensus rules for Section 3.6

zebra-chain/src/sapling/shielded_data.rs

@@ -290,6 +290,14 @@ where
     /// descriptions of the transaction, and the balancing value.
     ///
     /// <https://zips.z.cash/protocol/protocol.pdf#saplingbalance>
+    ///
+    /// # Consensus
+    ///
+    /// > The Spend transfers and Action transfers of a transaction MUST be
+    /// > consistent with its vbalanceSapling value as specified in § 4.13
+    /// > ‘Balance and Binding Signature (Sapling)’ on p. 49.
+    ///
+    /// <https://zips.z.cash/protocol/protocol.pdf#spendsandoutputs>
     pub fn binding_verification_key(&self) -> redjubjub::VerificationKeyBytes<Binding> {
         let cv_old: ValueCommitment = self.spends().map(|spend| spend.cv.into()).sum();
         let cv_new: ValueCommitment = self.outputs().map(|output| output.cv.into()).sum();

zebra-chain/src/sapling/spend.rs

@@ -171,6 +171,20 @@ impl ZcashSerialize for Spend<PerSpendAnchor> {
 }
 
 impl ZcashDeserialize for Spend<PerSpendAnchor> {
+    /// # Consensus
+    ///
+    /// > The anchor of each Spend description MUST refer to some earlier
+    /// > block’s final Sapling treestate. The anchor is encoded separately in
+    /// > each Spend description for v4 transactions, or encoded once and shared
+    /// > between all Spend descriptions in a v5 transaction.
+    ///
+    /// <https://zips.z.cash/protocol/protocol.pdf#spendsandoutputs>
+    ///
+    /// This rule is also implemented in
+    /// [`zebra_state::service::check::anchor`] and
+    /// [`zebra_chain::transaction::serialize`].
+    ///
+    /// The "anchor encoding for v4 transactions" is implemented here.
     fn zcash_deserialize<R: io::Read>(mut reader: R) -> Result<Self, SerializationError> {
         Ok(Spend {
             cv: commitment::NotSmallOrderValueCommitment::zcash_deserialize(&mut reader)?,

zebra-chain/src/transaction/serialize.rs

@@ -224,6 +224,21 @@ impl ZcashDeserialize for Option<sapling::ShieldedData<SharedAnchor>> {
             .collect();
 
         // Create transfers
+        //
+        // # Consensus
+        //
+        // > The anchor of each Spend description MUST refer to some earlier
+        // > block’s final Sapling treestate. The anchor is encoded separately
+        // > in each Spend description for v4 transactions, or encoded once and
+        // > shared between all Spend descriptions in a v5 transaction.
+        //
+        // <https://zips.z.cash/protocol/protocol.pdf#spendsandoutputs>
+        //
+        // This rule is also implemented in
+        // [`zebra_state::service::check::anchor`] and
+        // [`zebra_chain::sapling::spend`].
+        //
+        // The "anchor encoding for v5 transactions" is implemented here.
         let transfers = match shared_anchor {
             Some(shared_anchor) => sapling::TransferData::SpendsAndMaybeOutputs {
                 shared_anchor,