sub add_one_to_number() {
  # add_one_to_number ($number) : takes a number, returns number + 1
  # PRE1:   number exists
  # POST1:  we return the number plus one
  my $number = shift;
  if (! defined $number) { 
    return undef; # fail PRE1
  }
  $number++;
  return $number; # success POST1
}