USING: arrays combinators kernel make math math.combinatorics
math.ranges sequences sequences.deep ;

IN: combinatorics

: all-subsets ( seq -- subsets )
    dup length [0,b] [
        [ dupd all-combinations [ , ] each ] each
    ] { } make nip ;

: (selections) ( seq n -- selections )
    dupd [ dup 1 > ] [
        swap pick cartesian-product [
            [ [ dup length 1 > [ flatten ] when , ] each ] each
        ] { } make swap 1 -
    ] while drop nip ;

: selections ( seq n -- selections )
    {
        { 0 [ drop { } ] }
        { 1 [ 1array ] }
        [ (selections) ]
    } case ;