# reworked slightly, for conciseness.
sub new
{
  my %self;
  ( my $class, $self{qw( from_node to_node label )} ) = @_;
  bless \%self, $class;
}

##</code><code>##

sub new
{
  @_ == 2 and die "Error: can't set just one of the links!";
  my %self;
  ( my $class, $self{qw( from_node to_node label )} ) = @_;
  bless \%self, $class;
}

##</code><code>##

sub new
{
  my %self;
  ( my $class, $self{qw( from_node to_node label )} ) = @_;
  $self{'from_node'} && ! $self{'to_node'} and
    die "Error: can't set just one of the links!";
  bless \%self, $class;
}