Heap's Algorithm generates each permutation by swapping two elements of a
working array in place, and that working array was the argument itself, so
permutationsHeap([1, 2, 3]) left the caller holding [3, 2, 1]. Two calls on
one array returned different lists, and a frozen array threw.
Copy the input up front and swap on the copy, as shuffle and numericSort
already do. The returned permutations are unchanged.
dualfroz
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