Improve error handling in the eqn(7) parser.

Get rid of the first fatal error, MANDOCERR_EQNSYNT.
In eqn(7), there is no need to be bug-compatible with groff, so there
is no need to abondon the whole equation in case of a syntax error.

In particular:
* Skip "back", "delim", "down", "fwd", "gfont", "gsize", "left",
  "right", "size", and "up" without arguments.
* Skip "gsize" and "size" with a non-numeric argument.
* Skip closing delimiters that are not open.
* Skip "above" outside piles.
* For diacritic marks and binary operators without a left operand,
  default to an empty box.
* Let piles and matrices take one argument rather than insisting
  on a braced list.  Let HTML output handle that, too.
* When rewinding, if the root box is guaranteed to match
  the termination condition, no error handling is needed.

1 files changed, 1 insertions, 1 deletions

diff --git a/mandoc.h b/mandoc.h
index d18d5c97..bb8e51a2 100644
--- a/mandoc.h
+++ b/mandoc.h
@@ -104,6 +104,7 @@ enum	mandocerr {
 	MANDOCERR_BF_NOFONT, /* missing font type, using \fR: Bf */
 	MANDOCERR_BF_BADFONT, /* unknown font type, using \fR: Bf font */
 	MANDOCERR_ARG_STD, /* missing -std argument, adding it: macro */
+	MANDOCERR_EQN_NOBOX, /* missing eqn box, using "": op */
 
 	/* related to bad arguments */
 	MANDOCERR_ARG_QUOTE, /* unterminated quoted argument */
@@ -134,7 +135,6 @@ enum	mandocerr {
 	MANDOCERR_EQNSCOPE, /* equation scope open on exit */
 	MANDOCERR_EQNBADSCOPE, /* overlapping equation scopes */
 	MANDOCERR_EQNEOF, /* unexpected end of equation */
-	MANDOCERR_EQNSYNT, /* equation syntax error */
 
 	/* related to tables */
 	MANDOCERR_TBL, /* bad table syntax */