[xsl] All combinations from a sequence

["Michael Müller-Hillebrand mmh@xxxxxxxxx" <xsl-list-service@xxxxxxxxxxxxxxxxxxxxxx>]

Good afternoon,

I have a sequence of items and I need all combinations (not permutations) in
all possible lengths.

I saw what I want described as "powerset" in the Python docs:
powerset([1,2,3]) --> () (1,) (2,) (3,) (1,2) (1,3) (2,3) (1,2,3)

In XPath notation and based on strings:

my:powerset(('A','B','C','D'))

This sequence of 4 items should result in a sequence of 16 strings (order not
important) representing all possible combinations: 'ABCD', 'ABC', 'ABD',
'ACD', 'AB', 'AC', 'AD', 'A', 'BCD', 'BC', 'BD', 'B', 'CD', 'C', 'D', ''

Or more general, the result could be an array of sequences.

To get this as a solution in XSLT/XPath I am currently fiddling around with a
recursive function including head() and tail() and count() but I have the
impression I am overcomplicating things.

I am wondering, if this is a use case for fold-left() or if I should rather
think of a filter that drops 0, 1, 2 or 3 items from the sequence. Or is there
a well-known algorithm with a cool name?

Any hints are, as always, very welcome, thanks a lot,

- Michael