sub Listable::insert {
    my ($self, $child) = @_;
    
    die "Cannot insert ", ref($child), " into a ", ref($self)
        unless $self->is_legal_to_insert($child);

    push @{$self}, $child;
}

sub Listable::insert {
    return 1;
}

sub Matrices::is_legal_to_insert {
    my ($self, $child) = @_;
    return 1 if $child->isa( 'Matrix' );
    return $self->SUPER::is_legal_to_insert( $child );
}