COMBINE command syntax:

Valid map expressions (MAP_EXPR).  Note nesting capability
    (MAP_NAME)     (any valid map layer name)
    (name  new_map_name MAP_EXPR)
    (and   MAP_EXPR MAP_EXPR)
    ( or   MAP_EXPR MAP_EXPR)
    (group cat-nums MAP_EXPR )
    note: for "NOT":  (group 0 MAP_EXPR )
    (exp  number)

Commands and requests:
    quit
    help
    erase
    window
    window map_name
    statistics MAP_NAME
    categories MAP_NAME
      cats     MAP_NAME
    ! shell-command
    < filename
