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   groff_mm    ( 7 )

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Internals

The letter macros use different submacros depending on the letter
       type.  The name of the submacro has the letter type as suffix.
       It is therefore possible to define other letter types, either in
       the territory-specific macro file, or as local additions.  .LT
       sets the registers Pt and Pi to 0 and 5, respectively.  The
       following strings and macros must be defined for a new letter
       type.

       let@init_type
              This macro is called directly by .LT.  It is supposed to
              initialize registers and other stuff.

       let@head_type
              This macro prints the letter head, and is called instead
              of the normal page header.  It is supposed to remove the
              alias let@header, otherwise it is called for all pages.

       let@sg_type name title n flag [arg1 [arg2 [...]]]
              .SG is calling this macro only for letters; memorandums
              have its own processing.  name and title are specified
              through .WA/.WB.  n is the counter, 1-max, and flag is
              true for the last name.  Any other argument to .SG is
              appended.

       let@fc_type closing
              This macro is called by .FC, and has the formal closing as
              the argument.

       .LO is implemented as a general option-macro.  It demands that a
       string named Lettype is defined, where type is the letter type.
       .LO then assigns the argument to the string let*lo-type.