The follow-up loop could never fail: once the first "ar" index is asserted
to be 0, every non-"ar" element is necessarily at an index greater than 0.
A sequence like ["ar", "al", "ar"] passed both assertions, which is exactly
the interleaving the check exists to forbid.
Assert the partition directly instead: nothing after the first non-artist
call may be an artist. Verified by mutation — enqueueing artists a second
time after albums now fails the spec.
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