A BUILD_FAIL row now asserts the expected stderr substring on both stages (the #35 cite on the range form included) — without it a row passes vacuously on any unrelated build failure. The new delete_in_loop row drains [1,2,3,4] to empty via repeated delete(xs[0]) with base-4 positional accumulation, pinning the len bookkeeping under iteration. 38 -> 41 fixtures.