I find another proposed use of CODESEPARATOR here: https://lists.linuxfoundation.org/pipermail/lightning-dev/2016-March/000455.html <https://lists.linuxfoundation.org/pipermail/lightning-dev/2016-March/000455.html>

<KeyA> OP_CHECKSIG
OP_IF
<KeyB>
OP_ELSE
<Delay> OP_CSV OP_DROP
OP_CODESEPARATOR <KeyA>
OP_ENDIF
OP_CHECKSIG
It is actually 2 scripts:

S1: <KeyA> OP_CHECKSIGVERIFY <KeyB> OP_CHECKSIG
S2: <Delay> OP_CSV OP_DROP <KeyA> OP_CHECKSIG

Under taproot, we could make Q = P + H(P||S2)G, where P = MuSig(KeyA, KeyB)

S1 becomes a direct spending with Q, and there is no need to use OP_IF or CODESEPARATOR in S2 at all.


==================

If it is only to force R reuse, there is no need to use CODESEPARATOR:

Input: <R> <S2>  <S1>  Script: 2DUP EQUAL NOT VERIFY 2 PICK SWAP CAT <key> DUP TOALTSTACK CHECKSIGVERIFY CAT FROMALTSTACK CHECKSIG

But using CODESEPARATOR will save 3 bytes
Input: <S2> <R> <S1>    Script:  OVER SWAP CAT <key> DUP TOALTSTACK CHECKSIGVERIFY CODESEPARATOR SWAP CAT FROMALTSTACK CHECKSIG

However, a much better way would be:

Input: <S> Script: <known R> SWAP CAT <key> CHECKSIG

The discrete log of R could be a shared secret between A and B. If the purpose is to publish the private key to the whole world, R = G could be used.

> On 24 Dec 2018, at 8:01 PM, ZmnSCPxj <ZmnSCPxj@protonmail.com> wrote:
> 
> Good morning,
> 
>> Could anyone propose a better use case of CODESEPARATOR?
> 
> Long ago, aj sent an email on Lightning-dev about use of CODESEPARATOR to impose Scriptless Script even without Schnorr. It involved 3 signatures with different CODESEPARATOR places, and forced R reuse so that the signatures to claim the funds revealed the privkey.
> 
> The script shown had all CODESEPARATOR in a single branch.
> 
> I cannot claim to understand the script, and am having difficulty digging through the mailinglist
> 
> Regards,
> ZmnSCPxj