sub n_to_fb {
  my ( $n, $m ) = @_;
  my @fb;
  for my $i ( 2 .. $m ) {
    my $r = $n % $i;
    unshift @fb, $r;
    $n = int( $n/$i );
    ++$i;
  }
  return $n ? undef : \@fb;
}