The generator's own note says a real check-in DAG is acyclic by construction - a manifest's hash covers its P-card - while the phantom-parent flavor lets a hash be named before its record exists, so a later pick can close a loop and must be checked. The check only ever walked primary_parent_of, and the merge parent was drawn by pickInsertedIndex with the comment "no cycle risk".

That holds for the primary chain alone. Section 3.3 ancestry, which the B1 tie-break walks, follows merge parents too: a merge edge into an already inserted check-in, paired with a phantom primary edge out of it, closes a loop the guard never looked at. ancestryMaximal then finds every candidate dominated and no maximal element, and panics - seed 192 built exactly that, 9 to 14 to 13 and back.

parent_indices records every parent index a record names, wouldCloseCycle walks that graph visited-once, and the merge draw is checked like the primary one. A losing draw degrades to the single-parent record rather than drawing again, so the seeded stream shifts as little as it can.

Pre-existing: trunk fails seed 192 identically, and gen.zig is byte-identical there. Only the default six seeds hid it. check-in: 992ff19a23 user: dmestas tags: sub-manifest