sub _twig_extern_ent
   { # warn " in _twig_extern_ent...I (", $_[0]->original_string, ")\n"; # DEBUG handler
    my( $p, $base, $sysid, $pubid)= @_;
    my $t= $p->{twig};
    if( $t->{twig_no_expand}) 
      { my $ent_name= $t->{twig_keep_encoding} ? $p->original_string : $p->recognized_string;
        _twig_insert_ent( $t, $ent_name);
        return '';
      }
    my $ent_content= eval { $t->{twig_ext_ent_handler}->( $p, $base, $sysid) };
    if( ! defined $ent_content)
      { 
        my $ent_name = $p->recognized_string;
        my $file     =  _based_filename( $sysid, $base);
        my $error_message= "cannot expand $ent_name - cannot load '$file'";
        if( $t->{twig_extern_ent_nofail}) { return "<!-- $error_message -->"; }
        else                              { _croak( $error_message);   }
      }
    return $ent_content; 
  }