...
sub is_ancestor {
    my ( $self, $v1, $v2 ) = @_;
    for my $child ( $self->get_successors($v1) ) {
        return 1 if $child eq $v2; 
    }
    for my $child ( $self->get_successors($v1) ) {
        my $found = $self->is_ancestor( $child, $v2 );
        return 1 if $found;
    }
    return 0;
}

sub set_parents {
    my ( $self, $p1, $p2, $c, $checks ) = @_;
    $checks ||=1;

    if ( ( $self->in_degree($c) || 0) > 0) {
        croak "already has parents";
    }
    $self->add_edge_by_id($p1, $c, 1);
    if ($checks && $self->is_ancestor($c, $p1)) {
        $self->delete_edge_by_id($p1, $c, 1);
        croak 'Not a DAG anymore';
    }    
    $self->add_edge_by_id($p2, $c, 2);
    if ($checks && $p1 ne $p2 && $self->is_ancestor($c, $p2)) {
        $self->delete_edge_by_id($p2, $c, 2);
        croak 'Not a DAG anymore';
    }    
    return $self;
}