[lloeki]

> SET PC, [SP++] > SET PC, [--SP]

Good call, detecting (PC, SP) fixed point then, although using the stack to jump onto itself is wicked. I don't want to go and solve the halting problem either.

> SET PC, [location with I/O]

Qualifies as outside force. I'm not sure I want to write/run a program that JMP to an @ provided by I/O, this reads like an SQL injection.