# Takes a number and a list of elements.  Returns all possible ways to
# select that number out of the list.
sub choose_m {
  my $m = shift;
  # Special cases first
  if (0 == $m) {
    return [];
  }
  elsif (@_ < $m) {
    return ();
  }
  else {
    # This is our recursion step
    my $first = shift;
    return (
      # Things without the first element
      choose_m($m, @_),
      # Things with the first element
      map {[$first, @$_]} choose_m($m - 1, @_)
    );
  }
}