Append procedure

  DECLARATION:  Append(var F : Text);

      PURPOSE:  Opens an existing file for appending.

         UNIT:  System

      REMARKS:  Where F is a Text-File variable that must have been
                associated with an existing external file using Assign.

                If a CTRL-Z (ascii 26) is present in the last 128 byte
                block of the file, the current file position is set to
                overwrite the first CTRL-Z in the block.  This allows
                for text to be appended to a file that terminates with a
                CTRL-Z.

                If F was assigned an empty name, such as Assign(F,''),
                after the call to Append, F will refer to the standard
                output file (standard handle number 1).

                After a call to Append, F becomes Write-Only, and the
                pointer is positioned at the end of the file.

                NOTE: With {$I-}, errors are returned through IOResult