I think we need a way to allow more opcodes without taking up the rest of 
the NOPs.

This is related to a point from Murch 
(https://groups.google.com/g/bitcoindev/c/usHmnXDuJQc/m/hhtvAjSdCgAJ) that 
the reasoning of "its' compatible, why not" for adding 
CHECKSIGFROMSTACK(VERIFY/ADD) is not solid because when we add a new 
opcode, we usually have to give up a NOP. We do not have many NOPs left.

We can, however, solve that by allowing multi-byte opcodes.

Say, for example, we can have:
    OP_OP { 0x1521 }
which will set the current opcode to be the one with the assigned number 
0x1521.

Another idea is maybe OP_OP takes a stack element as the opcode.
    { 0x1521 } OP_OP

We can enforce some sort of minimal rule, or not do so, to allow more 
flexible use of existing opcodes.

This, of course, runs at a cost as this opcode needs three bytes in total 
to represent, but since the existing opcodes already take care of most of 
the basic functionalities that we expect users to use very frequently, the 
new opcodes that we want to add are likely those that complete something 
important and are going to be used only a few times in a script.

Similarly, we can require that multi-byte opcodes that have not been 
enabled my result in OP_SUCCESS.

OP_OP is not the best name as it could be confusing. OP_SETOP, OP_NEXT, etc 
could be taken into consideration.

The result of this is that we can worry less about whether it is worthy of 
a NOP to do an opcode, but focus on if the opcode has enough use cases to 
support it.

I feel that someone must have brought this up before (but it is a little 
bit hard to find the history in this mailing list at this moment).

What do people think?

Thanks,
Weikeng

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