DIRECT FILE ACCES COMMANDS - DISK(FILE) VARIABLES
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    General syntax is: <operation>$<variable>
                       <operation> can be put, close, \T
                       <variable> general syntax is:
                                  variable[,pos[,length]]  (no spacers are allowed like in 
                                                            schema-variable name;)
                                  if pos or length are variables, then 
                                     leading $-character in pos or length must be omitted;
                                     "variable or not" is recognized by the first char
                                          in the word pos or length;
                       f.e: "\T$dvar"         - takes 1 character from current position
                            "\T$dvar,where,"  - takes 1 character from $where position;
                            "\T$dvar,,2"      - takes 2 characters from current position;
                            in other words: pos has precedence before len,
                                            and empty parameter invokes defaults,
                                                but not $token;

    ..[r|w]open$variable.file_name  
          - open file_name and name it as "variable" for
            r - read,
            w - write, or
            both

    ..put$variable[,pos[,length]]."value"
          - put "value" into file <variable> from position pos;
          - if none of pos, len parameters is submitted, then
            "value" is put from the current file cursor position;
    ..close$variable
          - close file <variable>;  
            closing removes <variable> name from existing disk variables;
            closing non-opened variable generates exception with
            possible text processing termination;
    ..olen$variable[,assignee]
            length of open file corresponding to "variable"
            will be assigned to "assignee" or to token if 
            "assignee" is empty;

    in string:
      "....\T$variable[,[pos][,len]]<spacer>.... " 
          - retrieve value of file <variable> from 
            position pos, with length len
          - if none of parameters pos and len is submitted, then
            single character from current file cursor position
            is retrieved;
          - note: there is no spacer between $variable and first comma;




    NOTE: 1. <pos> and <len> can be variable names themselves:
             f.e.: \G$my_file,$pos_x,$len_y ...
          2. pos counted from 1;
          3. opening already opened disk-valiable with the new file name 
             closes opened file and opens file with the new name;
          4. disk-variables have own name space:
             no intersection with varibles name space;
          5. - for disk-variables opened for write (wopen),
                   "no_change_outside_result" is in effect, 
             - option "open" does not have "scope" protection,
                   "open" command must be used with care;