my $matrix_id = $cdat->add_matrix( $matrix );
my $tree_id   = $cdat->add_tree( $tree );
$cdat->add_handler(
    'listener'   => $tree_id,
    'observable' => $matrix_id,
    'handler'    => \&handler,
);
sub handler {
    my ( $listener, $observable, $method, @args ) = @_;
    # $method is called on $observable,
    # but supervisor $cdat creates an 
    # indirection layer to notify listeners
    print "$method '@args'" # prints "set_name 'New matrix'"
}
$matrix->set_name( 'New matrix' );