Fix some combinatorics #15293

Job	Run time
doctest (1.10, ubuntu-latest)	25m 15s
test (1.6, short, ubuntu-latest)	1h 7m 45s
doctest (1.11-nightly, ubuntu-latest)	23m 27s
test (1.6, long, ubuntu-latest)	39m 16s
doctest (nightly, ubuntu-latest)	22m 38s
doctest (1.10, ubuntu-latest, depwarn=error)	25m 40s
test (1.10, short, ubuntu-latest)	55m 34s
doctest (1.10, macOS, RPTU)	11m 28s
test (1.10, long, ubuntu-latest)	33m 13s
test (1.11-nightly, short, ubuntu-latest)	59m 26s
test (1.11-nightly, long, ubuntu-latest)	35m 38s
test (nightly, short, ubuntu-latest)	58m 37s
test (nightly, long, ubuntu-latest)	31m 33s
test (1.10, short, ubuntu-latest, depwarn=error)	51m 53s
test (1.10, long, ubuntu-latest, depwarn=error)	31m 32s
test (1.10, short, macOS, RPTU)	11m 17s
test (1.10, long, macOS, RPTU)	8m 23s
test (1.10, book, ubuntu-latest)	29m 37s
	10h 22m 12s

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