has 'id' =>  (
                is =>'ro',
                isa => 'Int',
                required => 1,
                builder => '_labelling',
                init_arg => undef,
            );
sub _labelling { $N++ }

has 'verlet' => (
                is =>'rw',
                isa => 'ArrayRef[Molecule::Atom]',
                required => 1,
                weak_ref   => 1,
                default => sub { [] },
);

sub add_verlet{
    my $self = shift;
    print $self->{id},"\n";
    my $array = $self->{verlet};
    foreach my $obj (@_){
            print "adding :", $obj->{id},"\n";
            push(@{$array},$obj);
            weaken(@{$array}[-1]);
    }
    print "Array dim = $#{$array}\n";
}

sub print_verlet{
    my $self = shift;
    my $array = $self->{verlet};
    print "Verlet from Atom ", $self->{id}, "\n";
    foreach my $obj (@{$array}){
        print "Verlet list :", $obj->{id},"---->\n";
    }
}