KOS_OP_CHECK_NIL

• Style: LABEL. Arg = target pc (post-compile).

• Stack: [..., x] → [...]. Pop TOS; branch to arg when it is not an empty-list singleton; fall through when it is.

• Invariants: Jump locality for target. The squashed form of NILP followed by JUMP_FALSE.

• Compiler form: Produced by Compiler.compress; pre-compile arg = string label, post-compile arg = pc.

• Implementation: koskari/lib/opcode/CHECK_NIL.c