Comment (by Erik Schnetter):

Yes, this code looks fishy. I looked for static variables, but I didn’t see this mutable one. Creating the iterator is expensive, hence this memoized variables.

The best way to rewrite this would be to add an explicit conversion (calling serialise) to a new type that has a cheap iterator (iter_memo_t). That requires rewriting all callers to begin and end, but that shouldn’t be too bad because it only makes expensive operations explicit.

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Ticket URL: https://bitbucket.org/einsteintoolkit/tickets/issues/2426/g-10-possibly-miscompiles-parts-of-carpet