sub set_error_callback
{
  my $self=shift;
  my $handler=shift // sub{};
  return 0 unless(ref($error) eq 'CODE');
  $self->{_error_handler}=$handler;
  return 1;
}

sub _set_error
{
  my $self=shift;
  my $error=shift // '';
  $self->{_error}=$error;
  $self->{_error_handler}->($self,$error) $self->{_error_handler};
}