Why does this return a Seq? I expected a List.

seq.p6

sub search(Int $x, Int $y, Int $max-depth is copy = 25000 --> List) {
  state %cache{Set};

  my @queue = (($x, $y, Nil, Nil),);
  while @queue and --$max-depth > 0 {
    my ($x, $y, $px, $py) = @queue.shift();

    %cache{Set($x, $y)} = [$px, $py];
    if $x == $y {
      my @path;
      while $x and $y {
        @path.push: [$x, $y];
        ($x, $y) = %cache{Set($x, $y)};
      }

      return reverse @path;
    }
    else {
      @queue.push: (2 * $x, $y + 1, $x, $y);
      @queue.push: ($x + 1, 2 * $y, $x, $y);
    }
  }

  return Nil;
}