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Mutation changes what an established iterator visits · case 04

Inserting, deleting or replacing source elements alters an iteration that promised a stable snapshot.

Member previewVariant 4 · 3 implementations · 7 checks per implementation

Case contract

Initial is a list of immutable scalar values. Edits are [yield index,operation,position,value] applied immediately before that snapshot yield. Operations append/delete/replace/insert/clear have valid positions at application time. Return [yielded original values,mutated source]. No yields means no scheduled edits execute. This is a shallow scalar snapshot, not object-graph isolation.

Why this case matters

Models stable observer iteration over a collection mutated by callbacks, with explicit separation between snapshot membership and the final state of the source collection.

One recorded failure

Sample boundary fixture

This sample comes from the broken implementation of a controlled reproducer.

Boundary fixture	Actual	Expected	Outcome
appended element is not visited	`[[0, 1, 2, 3, 4, 5, 99], [0, 1, 2, 3, 4, 5, 99]]`	`[[0, 1, 2, 3, 4, 5], [0, 1, 2, 3, 4, 5, 99]]`	Failed

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