--- a/cmd/isoareas/main.go	Wed Sep 25 15:39:55 2019 +0200
+++ b/cmd/isoareas/main.go	Wed Sep 25 17:25:49 2019 +0200
@@ -135,16 +135,20 @@
 	}
 
 	log.Println("inside classes:")
-	for _, c := range classes {
-		var eliminated int
+	for i, c := range classes {
+		remain := make([]int32, 0, len(c))
+
 		for _, idx := range c {
-			if inner(tri, idx, c) {
-				eliminated++
+			// Eliminate triangles that do not contribute
+			// any border to the final outline.
+			if !inner(tri, idx, c) {
+				remain = append(remain, idx)
 			}
 		}
 		log.Printf("\t%d (%d) %.2f%%\n",
-			len(c)-eliminated, len(c),
-			100*float64(eliminated)/float64(len(c)))
+			len(remain), len(c),
+			100*float64(len(remain))/float64(len(c)))
+		classes[i] = remain
 	}
 
 	log.Println("cuts:")
@@ -168,10 +172,9 @@
 			return false
 		}
 		n /= 3
-		p := sort.Search(len(indices), func(i int) bool {
+		if p := sort.Search(len(indices), func(i int) bool {
 			return indices[i] >= n
-		})
-		if p >= len(indices) || indices[p] != n {
+		}); p >= len(indices) || indices[p] != n {
 			return false
 		}
 	}