The .NUMBER CHAPTER command allows you to specify the number
    with which a sequence of chapters will begin. The next .CHAPTER
    command starts the sequence. Subsequent .CHAPTER commands will
    cause each chapter to be numbered one higher than the previous
    chapter. (See also .DISPLAY CHAPTER.)

    Format

      .NUMBER CHAPTER  [[+/-]n]

    Abreviated format

      .NMCH  [[+/-]n]