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Generate all permuations of the numbers 1, 2, ..., NPERM;
Ref:
Heap, B. R. (1963). "Permutations by Interchanges". The Computer Journal. 6 (3): 293–4..
Sedgewick, R. (1977). "Permutation Generation Methods". ACM Computing Surveys. 9 (2):
137–164.;