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, 8 insertions, 4 deletions

diff --git a/eqn_html.c b/eqn_html.c
index 22881b72..f4bd3482 100644
--- a/eqn_html.c
+++ b/eqn_html.c
@@ -47,10 +47,12 @@ eqn_box(struct html *p, const struct eqn_box *bp)
 	if (EQN_MATRIX == bp->type) {
 		if (NULL == bp->first)
 			goto out;
-		assert(EQN_LIST == bp->first->type);
+		if (EQN_LIST != bp->first->type) {
+			eqn_box(p, bp->first);
+			goto out;
+		}
 		if (NULL == (parent = bp->first->first))
 			goto out;
-		assert(EQN_PILE == parent->type);
 		/* Estimate the number of rows, first. */
 		if (NULL == (child = parent->first))
 			goto out;
@@ -126,8 +128,10 @@ eqn_box(struct html *p, const struct eqn_box *bp)
 
 	if (EQN_PILE == bp->type) {
 		assert(NULL == post);
-		post = print_otag(p, TAG_MTABLE, 0, NULL);
-	} else if (bp->parent && EQN_PILE == bp->parent->type) {
+		if (bp->first != NULL && bp->first->type == EQN_LIST)
+			post = print_otag(p, TAG_MTABLE, 0, NULL);
+	} else if (bp->type == EQN_LIST &&
+	    bp->parent && bp->parent->type == EQN_PILE) {
 		assert(NULL == post);
 		post = print_otag(p, TAG_MTR, 0, NULL);
 		print_otag(p, TAG_MTD, 0, NULL);