Let's try this. It's BUGGERED.

src/cmd/tbl/ti.c

@@ -0,0 +1,75 @@
+/* ti.c: classify line intersections */
+# include "t.h"
+/* determine local environment for intersections */
+
+int
+interv(int i, int c)
+{
+	int	ku, kl;
+
+	if (c >= ncol || c == 0) {
+		if (dboxflg) {
+			if (i == 0) 
+				return(BOT);
+			if (i >= nlin) 
+				return(TOP);
+			return(THRU);
+		}
+		if (c >= ncol)
+			return(0);
+	}
+	ku = i > 0 ? lefdata(i - 1, c) : 0;
+	if (i + 1 >= nlin && allh(i))
+		kl = 0;
+	else
+		kl = lefdata(allh(i) ? i + 1 : i, c);
+	if (ku == 2 && kl == 2) 
+		return(THRU);
+	if (ku == 2) 
+		return(TOP);
+	if (kl == BOT) 
+		return(2);
+	return(0);
+}
+
+
+int
+interh(int i, int c)
+{
+	int	kl, kr;
+
+	if (fullbot[i] == '=' || (dboxflg && (i == 0 || i >= nlin - 1))) {
+		if (c == ncol)
+			return(LEFT);
+		if (c == 0)
+			return(RIGHT);
+		return(THRU);
+	}
+	if (i >= nlin) 
+		return(0);
+	kl = c > 0 ? thish (i, c - 1) : 0;
+	if (kl <= 1 && i > 0 && allh(up1(i)))
+		kl = c > 0 ? thish(up1(i), c - 1) : 0;
+	kr = thish(i, c);
+	if (kr <= 1 && i > 0 && allh(up1(i)))
+		kr = c > 0 ? thish(up1(i), c) : 0;
+	if (kl == '=' && kr ==  '=') 
+		return(THRU);
+	if (kl == '=') 
+		return(LEFT);
+	if (kr == '=') 
+		return(RIGHT);
+	return(0);
+}
+
+
+int
+up1(int i)
+{
+	i--;
+	while (instead[i] && i > 0) 
+		i--;
+	return(i);
+}
+
+