sub extract (&\@) {
  my ($code, $aref) = @_;
  my @return;
  my @replace;
  for (@$aref) { # aliases $_ to the item in the original list
    if (&$code) {
      push @return, $_;
    } else {
      push @replace, $_;
    }
  }
  @$aref = @replace;
  return @return;
}