OP_CAT
Introduction
OP_CAT is a simple opcode that concatenates two byte strings together in Bitcoin Script.
Andrew Poelstra once wrote a two-part series of blog posts to showcase how the OP_CAT opcode can
in itself effectively enable covenants:
It's important to clarify that this proposal doesn't aim to introduce covenants to Bitcoin. Instead,
it is focused on introducing an opcode that holds broad utility across various scenarios. The
potency of several covenant proposals increases substantially when they can be integrated with the
functionality provided by CAT.
As a side note, OP_CAT is also one of the opcodes that would be re-enabled as part of the Great
Script Restoration project.
Specification
A BIP of the OP_CAT opcode is currently in draft status on
GitHub.
An implementation on Bitcoin Inquisition is also available.