Re: file variables and man pages with preprocessors

[Richard Stallman]

    Well, preconv acts as a filter -- as do (almost) all components in
    groff -- thus reading something from the `end of a file' (a) needs a
    temporary buffer since the input data stream is `stdin' in most cases
    and (b) isn't implemented in preconv due to (a).

I don't understand this.  What is preconv, and what does it have to do
with visiting these files in Emacs?