function  __attrfilter(fileattr, filter: byte): boolean;

   Selects whether a file is admissible.
   Examples:
        archive + hidden + readonly + sysfile + directory + volumid + only +
     1 finding all normal files
       if __attrfilter(
         search.attr,
         archive + readonly
       ) then writeln('Found: ', search.name);
       Files that are found can have: none, arc, r/o or arc + r/o attributes!

     2 finding all system files files
       if __attrfilter(
         search.attr,
         hidden + sysfile + only
       ) then writeln('Found: ', search.name);
       Files that are found can have only hidden + sysfile attributes!

     3 finding all system files files
       if __attrfilter(
         search.attr,
         volumid  directory + only + notnone
       ) then writeln('Found: ', search.name);
       Files that are found cannot have no attributes since
       "notnone" is specified.
       Furthermore they can have directory or volumid attributes.
       By error, since DOS normally does not allow it, files
       with both VolumID and Directory attributes are found, too.