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(groff)Preprocessor Intro

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Preprocessors
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   Although `groff' provides most functions needed to format a
document, some operations would be unwieldy (e.g. to draw pictures).
Therefore, programs called preprocessors were written which understand
their own language and produce the necessary `groff' operations.  These
preprocessors are able to differentiate their own input from the rest
of the document via markers.

   To use a preprocessor, UNIX pipes are used to feed the output from
the preprocessor into `groff'.  Any number of preprocessors may be used
on a given document; in this case, the preprocessors are linked
together into one pipeline.  However, in `groff', the user does not
need to construct the pipe, but only tell `groff' what preprocessors to
use.

   `groff' currently has preprocessors for producing tables (`tbl'),
typesetting equations (`eqn'), drawing pictures (`pic' and `grn'), and
for processing bibliographies (`refer').  An associated program which
is useful when dealing with preprocessors is `soelim'.

   A free implementation of `grap', a preprocessor for drawing graphs,
can be obtained as an extra package; `groff' can use `grap' also.

   There are other preprocessors in existence, but, unfortunately, no
free implementations are available.  Among them are preprocessors for
drawing mathematical pictures (`ideal') and chemical structures
(`chem').