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+.so g.tmac
+.
+.de @h
+. sp |36p
+. if \\n%>1 .tl ''-%-''
+. sp |1i
+..
+.de @f
+.sp |\\n(.pu-36p
+.tl ''\\*(#e''
+..
+.
+.t
+Hacking on the
+.x mg
+macro package
+.d
+John Ankarstr\[:o]m
+.
+.
+.h
+What is
+.x mg ?
+.p
+.i Mg
+is a simple macro package for troff
+designed to abstract as little as possible from troff itself,
+while still providing a powerful framework for
+writing advanced documents.
+.
+.
+.h
+How is the source code of
+.x mg
+organized?
+.p
+If you run
+.c "grep -n [-]-"
+on the
+.i g.tmac
+source file, you are presented with an overview of
+.i mg 's
+macros:
+.l
+.eo
+!grep -n [-]- g.tmac
+.ec
+.p
+This is a sufficient summary of the entire
+.i mg
+source code, as nothing is performed outside of these macros.
+All initialization is performed in the
+.c @a
+macro, which is automatically called at the first invocation
+of any other macro.
+.p
+The above summary reflects a categorization in the macros defined by
+.i mg .
+There are internal and external macros.
+The former are to be used within
+.i g.tmac
+itself, while the latter are to be used in
+.i mg
+documents.
+Among the external macros, there are inline, environment (or block-level),
+hybrid and other macros.
+.p
+The inline macros all follow the same pattern.
+They take three arguments:
+the string to be formatted,
+an optional suffix
+and an optional prefix.
+The hybrid macros act as inline macros when given arguments;
+otherwise they act as environment macros.
+.p
+The environment or block-level macros generally take no arguments
+(except
+.c d ).
+Instead, they activate a given environment,
+affecting the formatting of the following text.
+Each environment macro is associated with a specific environment,
+carrying the same one-letter name as the macro itself.
+.p
+As you can see,
+the macros in each category are arranged alphabetically.
+.
+.
+.h
+Where is document state stored?
+.p
+Most state is stored by troff itself within the different environments.
+In addition,
+.i mg
+associates three extra registers with each environment:
+.c sp ,
+the amount of space to be added by
+.c @e
+before an environment;
+.c sq ,
+the same (except the space is not added
+if the new environment is identical to the previous one); and
+.c ti ,
+the indentation of the first line in the
+.c p
+environment.
+These are stored in registers named
+.c @ENV_sp ,
+.c @ENV_sq
+and
+.c @ENV_ti ,
+where
+.c ENV
+is the name of the associated environment.
+.p
+The strings
+.c %env
+and
+.c %penv
+contain the name of the current and previous environment.
+.p
+The
+.c @a
+register is set to 1 if the document has been initialized
+(i.e. if
+.c @a
+has been invoked).
+.p
+The
+.c @m
+register is non-zero if
+.q "manual footer"
+mode is active.
+If
+.c @m
+is non-zero,
+.c @tf
+decrements it by one and exits when invoked,
+unless called with the
+.c f
+(force) argument.
+This is useful if you want to trigger the footer manually,
+but do not want the printed footer to trigger the footer trap again.
+.p
+.c @.t
+contains the absolute vertical position of the first trap
+following the first footnote reference on a page;
+it is set and used by
+.c )
+to place the footnote trap in the correct vertical position.
+.c @dn
+contains the height of all collected footnotes on a page;
+it is set by
+.c )
+and reset to zero by
+.c @tn .
+.c @n
+contains the total number of collected footnotes.
+.p
+Note that none of these registers and strings should be
+directly accessed or modified by
+.i mg
+documents.