[Python-ideas] Extremely weird itertools.permutations

[Tim Peters]

One example of prior art:  Maxima, which I use in its wxMaxima incarnation.

"""
Function:  permutations(a)

Returns a set of all distinct permutations of the members of the list
or set a. Each permutation is a list, not a set.

When a is a list, duplicate members of a are included in the permutations
"""

Examples from a Maxima shell:

> permutations([1, 2. 3]);
{[1,2,3],[1,3,2],[2,1,3],[2,3,1],[3,1,2],[3,2,1]}

> permutations([[1, 2], [1, 2], [2, 3]])
{[[1,2],[1,2],[2,3]],
 [[1,2],[2,3],[1,2]],
 [[2,3],[1,2],[1,2]]}

> permutations({1, 1.0, 1, 1.0})
{[1,1.0],[1.0,1]}

That last one may be surprising at first, but note that it's the first
example where I passed a _set_ (instead of a list).  And:

> {1, 1.0, 1, 1.0}
{1,1.0}

Best I can tell, Maxima has no builtin function akin to our
permutations(it, r) when r < len(it).  But Maxima has a huge number of
builtin functions, and I often struggle to find ones I _want_ in its
docs ;-)