0 = 0b0000 --> {nothing}
 3 = 0b0011 --> 'horse:cow'
13 = 0b1101 --> 'horse:dog:cat'

##</code><code>##

sub rhesa {
    # initial source in sequence order
    my $source = 'horse:cow:dog:cat';

    my @kw = split /:/, $source;
    my @res;
    
    for my $i( 1 .. 2**@kw - 1 ) {
        my @ar; my $t;
        while( $i > 0 ) {
            push @ar, $kw[$t] if $i & 1;
            $i >>= 1; $t++;
        }
        push @res, join ':', @ar;
    }

    return @res;
}