HH\ImmSet::zip

Throws an exception unless the current ImmSet or the Traversable is empty

public function zip<Tu>(
  Traversable<Tu> $traversable,
): ImmSet<Pair<Tv, Tu>>;

Since ImmSets only support integers or strings as values, we cannot have a Pair as an ImmSet value. So in order to avoid an InvalidArgumentException, either the current ImmSet or the Traversable must be empty so that we actually return an empty ImmSet.

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