bip54: clarify sigops counting, borrow bip16 language

Author: instagibbs

bip-0054.md

@@ -69,12 +69,16 @@ Given a block at height `N`:
 A limit is set on the number of potentially executed signature operations in validating a
 transaction. It applies to all transactions in the block except the coinbase transaction[^1]. For
 each input in the transaction, count the number of `CHECKSIG` and `CHECKMULTISIG` in the input
-scriptSig and previous output's scriptPubKey, including the P2SH redeemScript. The accounting is the
-same as for [bip-0016][BIP16 specs]: a `CHECKSIG`/`CHECKSIGVERIFY` operation accounts for 1 and a
-`CHECKMULTISIG`/`CHECKMULTISIGVERIFY` accounts for the number of public keys associated, or 20 if
-the number of public keys is greater than 16. A `CHECKMULTISIG`/`CHECKMULTISIGVERIFY` not directly
-preceded by a minimally-pushed number between 1 and 16 (included) accounts for 20. If the
-total is strictly higher than 2500, the transaction is invalid.
+scriptSig and previous output's scriptPubKey, including the P2SH redeemScript. If the total summed
+over all transaction inputs is strictly higher than 2500, the transaction is invalid. The accounting is the
+same as for [bip-0016][BIP16 specs], evaluating the scriptSig, scriptPubKey, and P2SH redeemScript
+separately:
+
+1. `CHECKSIG` and `CHECKSIGVERIFY` count as 1 signature operation, whether or not they are evaluated.
+2. `CHECKMULTISIG` and `CHECKMULTISIGVERIFY` immediately preceded by OP_1 through OP_16 are counted as 1 to 16 signature operations, whether or not they are evaluated.
+3. All other `CHECKMULTISIG` and `CHECKMULTISIGVERIFY` are counted as 20 signature operations, whether or not they are evaluated.
+
+If the total summed over all transaction inputs is strictly higher than 2500, the transaction is invalid.
 
 Transactions whose witness-stripped serialized size is exactly 64 bytes are invalid.