be slightly more explicit about codesep_pos

Felt this could be under-defined, and if this is wrong, would be nice to have correct text in its place

Author: instagibbs

bip-0342.mediawiki

@@ -110,7 +110,7 @@ To validate a signature ''sig'' with public key ''p'':
 * Compute the tapscript message extension ''ext'', consisting of the concatenation of:
 ** ''tapleaf_hash'' (32): the tapleaf hash as defined in [[bip-0341.mediawiki#design|BIP341]]
 ** ''key_version'' (1): a constant value ''0x00'' representing the current version of public keys in the tapscript signature opcode execution.
-** ''codesep_pos'' (4): the opcode position of the last executed <code>OP_CODESEPARATOR</code> before the currently executed signature opcode, with the value in little endian (or ''0xffffffff'' if none executed). The first opcode in a script has a position of 0. A multi-byte push opcode is counted as one opcode, regardless of the size of data being pushed.
+** ''codesep_pos'' (4): the opcode position of the last executed <code>OP_CODESEPARATOR</code> before the currently executed signature opcode, with the value in little endian (or ''0xffffffff'' if none executed). The first opcode in a script has a position of 0. A multi-byte push opcode is counted as one opcode, regardless of the size of data being pushed. Opcodes in parsed but unexecuted branches count towards this value as well.
 * If the ''sig'' is 64 bytes long, return ''Verify(p, hash<sub>TapSighash</sub>(0x00 || SigMsg(0x00, 1) || ext), sig)'', where ''Verify'' is defined in [[bip-0340.mediawiki#design|BIP340]].
 * If the ''sig'' is 65 bytes long, return ''sig[64] &ne; 0x00 and Verify(p, hash<sub>TapSighash</sub>(0x00 || SigMsg(sig[64], 1) || ext), sig[0:64])''.
 * Otherwise, fail.