KOS_OP_MATCH_AT

• Style: EXTRA (RAW): mark depth.

• Stack: [...] -> [..., e]; cannot miss.

• Semantics: Push the element at the position of the mark at depth a0, as MATCH_NEXT gave it. Raises ValueError when the subject no longer holds that element: a list shrunk below the position, or a spine cell that is no longer a pair.

• Compiler form: Emitted by the match compiler for a capture over a pattern one element wide.