The .HEADER LEVEL command allows you to specify both a section
    number and a section title. Successive .HEADER LEVEL commands
    of the same value (all .HEADER LEVEL 1's for example) cause the
    section numbers to increase sequentially. This happens at all six
    levels of headers. If your current section is in Chapter 2 and is
    numbered 2.5.2.4 (a header level 3), then the following numbering
    would result depending upon the .HEADER LEVEL command you used:

    o  .HL4 would number the next section 2.5.2.4.1

    o  .HL3 (or .HL without a value, since .HL defaults to the
       current level) would number the next section 2.5.2.5

    o  .HL2 would number the next section 2.5.3

    o  .HL1 would number the next section 2.6

    (See also .DISPLAY LEVELS, .NUMBER LEVEL, .SET LEVEL, and .STYLE
    HEADERS.)

    Following is a summary of default header level numbering for
    three levels of three different types of documents:
    Default Header Level Numbering

                       Nonchapter  Chapter n    Appendix A

    .HEADER LEVEL 1    1           n.1          A.1
    .HEADER LEVEL 2    1.1         n.1.1        A.1.1
    .HEADER LEVEL 3    1.1.1       n.1.1.1      A.1.1.1

    Format

      .HEADER LEVEL  [[+/-]n] [title]

    Abreviated format

      .HL  [[+/-]n] [title]