sub list {
    my $self = shift @_;
    my $key  = shift @_;
    my $aref = $self->{$key};

    return wantarray ? @$aref : $aref;
}

##</code><code>##

    my @population = $self->list("_population");

##</code><code>##

sub remove {
    my $self   = shift;
    my $type   = shift @_;
    my $target = shift @_;

    $self->{$type} = [ grep { $target ne $_ } @{$self->{$type}} ];
    return;
}