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Good catch.
CALL_FUNCTION seems to split its opcode into two to give it a positional-keyword pair so this seems fine. I'd hope we can do the same thing; personally I would do:
 BUILD_MAP_UNPACK(
 position_of_function_in_stack_or_0 << 8 |
 number_to_pack
 )
This way if building for a function we can do the check *and* give good errors that match the ones raised from CALL_FUNCTION. When the top 8 bits are 0, we don't do checks. What do you think? Would dual-usage be too confusing?