LOCK...UNLOCK - file I/O statements that control access by other
                processes to all or part of an opened file
 
Tip: LOCK and UNLOCK are only useful in network environments where
     several processes my need access to the same file, and only
     function if you are using DOS version 3.1 or later.
 
Syntax
  LOCK [#]filenumber [,{record |[start] TO end}]
  [statements]
  UNLOCK [#]filenumber [,{record | [start] TO end}]
 
    - filenumber is the number of an open file
    - record, start, and end indicate the range of records or bytes to
      be locked