> Step 2.
>
> This finishes off the new escape-sequence parser and puts it into
> mandoc_escape (mandoc.c, mandoc.h), then makes it the underlying engine
> for a2roffdeco (out.c) (requiring a tiny change to term.c and html.c for
> bailing out on bad sequences) and roff_res (roff.c).
>
> Now all escape-sequences are being parsed with the same engine! This
> logic was being repeated in THREE different places, earlier (mandoc.c
> for validation, out.c for output, and roff.c for predefined escapes).
>
> I've run this over all manuals I know of without problems, but it can
> really use a close look-over with border cases.
>
> The next step is to clean out the out.c code, completely removing enum
> roffdeco (putting that logic into mandoc.c, perhaps).

Step 2b.  I rolled back the roff part: the search/replace of predefined 
strings must happen prior to escape processing.  I'd also forgotten to 
include mdoc_validate.c in the patch.