FILE SYSEM COMMANDS
===================

   ..save[$[variable]].file_name      
   ..read[$[variable]].file_name
   ..include.file_name
   ..mkdir.file_name
        
        where file_name is
           absolute_path (which means including drive specification like c:...),
           or if file_name is not an absolute path, then
              path relative to to currently processing source unit or file - for read and include
              path relative to of currently processing result unit or file - for save and mkdir
        file name can be variable or escaped-string;

   ..flen    "FileLength" - length of not open file.
             syntax ..FLEN[$[var].file_name
               if no $      => var = token
               empty var    => var = token
             file_name - variable or string-expression;