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

sub Foo::is_legal_to_insert {
    my ($self, $child) = @_;
    return 0 if $child->isa( 'Bar' );
    return $self->SUPER::is_legal_to_insert( $child );
}