Let's try this. It's BUGGERED.

src/cmd/tbl/tt.c

@@ -0,0 +1,127 @@
+/* tt.c: subroutines for drawing horizontal lines */
+# include "t.h"
+
+int
+ctype(int il, int ic)
+{
+	if (instead[il])
+		return(0);
+	if (fullbot[il])
+		return(0);
+	il = stynum[il];
+	return(style[ic][il]);
+}
+
+
+int
+min(int a, int b)
+{
+	return(a < b ? a : b);
+}
+
+
+int
+fspan(int i, int c)
+{
+	c++;
+	return(c < ncol && ctype(i, c) == 's');
+}
+
+
+int
+lspan(int i, int c)
+{
+	int	k;
+
+	if (ctype(i, c) != 's') 
+		return(0);
+	c++;
+	if (c < ncol && ctype(i, c) == 's')
+		return(0);
+	for (k = 0; ctype(i, --c) == 's'; k++)
+		;
+	return(k);
+}
+
+
+int
+ctspan(int i, int c)
+{
+	int	k;
+	c++;
+	for (k = 1; c < ncol && ctype(i, c) == 's'; k++)
+		c++;
+	return(k);
+}
+
+
+void
+tohcol(int ic)
+{
+	if (ic == 0)
+		Bprint(&tabout, "\\h'|0'");
+	else
+		Bprint(&tabout, "\\h'(|\\n(%2su+|\\n(%2su)/2u'", reg(ic, CLEFT),
+		     reg(ic - 1, CRIGHT));
+}
+
+
+int
+allh(int i)
+{
+			/* return true if every element in line i is horizontal */
+				/* also at least one must be horizontl */
+	int	c, one, k;
+
+	if (fullbot[i]) 
+		return(1);
+	if (i >= nlin) 
+		return(dboxflg || boxflg);
+	for (one = c = 0; c < ncol; c++) {
+		k = thish(i, c);
+		if (k == 0) 
+			return(0);
+		if (k == 1) 
+			continue;
+		one = 1;
+	}
+	return(one);
+}
+
+
+int
+thish(int i, int c)
+{
+	int	t;
+	char	*s;
+	struct colstr *pc;
+
+	if (c < 0)
+		return(0);
+	if (i < 0) 
+		return(0);
+	t = ctype(i, c);
+	if (t == '_' || t == '-')
+		return('-');
+	if (t == '=')
+		return('=');
+	if (t == '^') 
+		return(1);
+	if (fullbot[i] )
+		return(fullbot[i]);
+	if (t == 's') 
+		return(thish(i, c - 1));
+	if (t == 0) 
+		return(1);
+	pc = &table[i][c];
+	s = (t == 'a' ? pc->rcol : pc->col);
+	if (s == 0 || (point(s) && *s == 0))
+		return(1);
+	if (vspen(s)) 
+		return(1);
+	if (t = barent( s))
+		return(t);
+	return(0);
+}
+
+