sub combinations returns Array  (@list is rw) {
  return () unless @list.elems;
  gather {
      for 1 .. 2**@list.elems-1-> $num {
          take [ @list[ (0 .. sqrt($num)).grep:{ $num +& (2**$_) } ] ];
      }
  }
} 
my @list = (1..4);
combinations(@list).perl.say;