$tree->insert($node); #works

$matrix->insert($datum); #works

$trees->insert($node); #error: $trees object can't contain $node

$matrix->insert($node); #error: $matrix object can't contain $node

##</code><code>##

sub insert {
    my ( $self, $obj_to_insert ) = @_;
    if ( $self->container_type eq $obj_to_insert->container ) {
        push(@{$self}, $obj_to_insert);
    }
    else {
        die "I can't hold that object!";
    }
}