SET(OVERSTRIKE)
  Sets or changes the mode for entering text in the specified buffer to
  overstrike.  New characters replace existing characters starting at the
  current position.  The default setting is insert.
  Syntax
     SET (OVERSTRIKE, buffer)
  Example
     SET (OVERSTRIKE, CURRENT_BUFFER);
  Sets the mode for entering text in the current buffer to overstrike.
  Related topics
     COPY_TEXT   MOVE_TEXT   SET(INSERT)