Replacing a list by a list containing each of its distinct elements once. Retaining first occurrences gives a stable order. A recursive implementation emits the head and removes all its occurrences from the tail before continuing. On an infinite input with only finitely many distinct values, detecting that there are no further new values may diverge; on an infinite input with infinitely many distinct values and terminating equality tests, each successive new value has a finite position and can be emitted.
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