Thanks! It is derived PDP-8's own OPR instruction, which goes even further, with many carefully aligned combinations leading to clever results. A testament of ingenuity which reveals fine craftsmanship.

In reality, I've considered switching to a regular mapping, so to have 32 no-operand instructions instead, which would fit the original five operations, all sensible combinations, plus more.

But chaining operands is definitely a more extravagant thing, so I think it can be instructive in showing how creative a solution can be in face of limitations .