*powerset = pipeline(
        map_c { [ [$_], [] ] },              # step 1
        \&combos,                            # step 2
        map_c { map [ map @$_, @$_ ], @$_ }  # step 3
    );

    say powerset( 1, 2 );
    # [1,2] [1] [2] []

    say powerset(qw( a b c ));
    # ['a','b','c'] ['a','b'] ['a','c'] ['a']
    # ['b','c'] ['b'] ['c'] []