Index: usr.bin/diff3/diff3.c
===================================================================
--- usr.bin/diff3/diff3.c
+++ usr.bin/diff3/diff3.c
@@ -112,6 +112,11 @@
 	struct range new;
 };
 
+#define NOEFLAG 	0
+#define EFLAG_OVERLAP 	1
+#define EFLAG_NOOVERLAP	2
+#define EFLAG_UNMERGED	3
+
 static size_t szchanges;
 
 static struct diff *d13;
@@ -314,7 +319,7 @@
 		/* first file is different from the others */
 		if (!t2 || (t1 && d1->new.to < d2->new.from)) {
 			/* stuff peculiar to 1st file */
-			if (eflag == 0) {
+			if (eflag == NOEFLAG) {
 				printf("====1\n");
 				change(1, &d1->old, false);
 				keep(2, &d1->new);
@@ -325,7 +330,7 @@
 		}
 		/* second file is different from others */
 		if (!t1 || (t2 && d2->new.to < d1->new.from)) {
-			if (eflag == 0) {
+			if (eflag == NOEFLAG) {
 				printf("====2\n");
 				keep(1, &d2->new);
 				change(3, &d2->new, false);
@@ -362,7 +367,7 @@
 			 * dup = 0 means all files differ
 			 * dup = 1 means files 1 and 2 identical
 			 */
-			if (eflag == 0) {
+			if (eflag == NOEFLAG) {
 				printf("====%s\n", dup ? "3" : "");
 				change(1, &d1->old, dup);
 				change(2, &d2->old, false);
@@ -532,7 +537,15 @@
 static int
 edit(struct diff *diff, bool dup, int j, int difftype)
 {
-
+	/*
+	 * dup is promoted here to an integer and evaluates to 2 if dup was
+	 * true or 1 if dup was false and then compared against the eflag mode,
+	 * don't take the edit if:
+	 * - EFLAG_OVERLAP (01): if we are printing overlaps and this was not
+	 *   dup'd (i.e. in both and overlapping)
+	 * - EFLAG_NOOVERLAP (10): if we are not printing overlaps and this is
+	 *   dup'd (i.e. in both)
+	 */
 	if (((dup + 1) & eflag) == 0)
 		return (j);
 	j++;
@@ -609,7 +622,7 @@
 	if (iflag)
 		printf("w\nq\n");
 
-	exit(eflag == 0 ? overlapcnt : 0);
+	exit(eflag == NOEFLAG ? overlapcnt : 0);
 }
 
 /*
@@ -827,13 +840,13 @@
 	struct kevent *e;
 
 	nblabels = 0;
-	eflag = 0;
+	eflag = NOEFLAG;
 	oflag = 0;
 	diffargv[diffargc++] = __DECONST(char *, diffprog);
 	while ((ch = getopt_long(argc, argv, OPTIONS, longopts, NULL)) != -1) {
 		switch (ch) {
 		case '3':
-			eflag = 2;
+			eflag = EFLAG_NOOVERLAP;
 			break;
 		case 'a':
 			diffargv[diffargc++] = __DECONST(char *, "-a");
@@ -842,10 +855,10 @@
 			Aflag = 1;
 			break;
 		case 'e':
-			eflag = 3;
+			eflag = EFLAG_UNMERGED;
 			break;
 		case 'E':
-			eflag = 3;
+			eflag = EFLAG_UNMERGED;
 			oflag = 1;
 			break;
 		case 'i':
@@ -866,11 +879,11 @@
 			Tflag = 1;
 			break;
 		case 'x':
-			eflag = 1;
+			eflag = EFLAG_OVERLAP;
 			break;
 		case 'X':
 			oflag = 1;
-			eflag = 1;
+			eflag = EFLAG_OVERLAP;
 			break;
 		case DIFFPROG_OPT:
 			diffprog = optarg;
@@ -891,7 +904,7 @@
 	argv += optind;
 
 	if (Aflag) {
-		eflag = 3;
+		eflag = EFLAG_UNMERGED;
 		oflag = 1;
 	}