diff --git a/gen.go b/gen.go
index 224eef9..51e6603 100644
--- a/gen.go
+++ b/gen.go
@@ -34,6 +34,7 @@
 	verbose     = flag.Bool("v", false, "verbose output")
 	force       = flag.Bool("force", false, "ignore failing dependencies")
 	doCore      = flag.Bool("core", false, "force an update to core")
+	skipTest    = flag.Bool("skiptest", false, "skip tests")
 	excludeList = flag.String("exclude", "",
 		"comma-separated list of packages to exclude")
 
@@ -209,6 +210,10 @@
 			return
 		}
 
+		if *skipTest {
+			return
+		}
+
 		vprintf("=== TEST %s\n", pkg)
 		args[0] = "test"
 		cmd = exec.Command(filepath.Join(runtime.GOROOT(), "bin", "go"), args...)
diff --git a/internal/gen/code.go b/internal/gen/code.go
index 8622c7f..55dd1ed 100644
--- a/internal/gen/code.go
+++ b/internal/gen/code.go
@@ -66,7 +66,9 @@
 func (w *CodeWriter) WriteVersionedGoFile(filename, pkg string) {
 	tags := buildTags()
 	if tags != "" {
-		filename = insertVersion(filename, UnicodeVersion())
+		pattern := fileToPattern(filename)
+		updateBuildTags(pattern)
+		filename = fmt.Sprintf(pattern, UnicodeVersion())
 	}
 	f, err := os.Create(filename)
 	if err != nil {
diff --git a/internal/gen/gen.go b/internal/gen/gen.go
index 13257d6..41c3268 100644
--- a/internal/gen/gen.go
+++ b/internal/gen/gen.go
@@ -31,6 +31,7 @@
 	"os"
 	"path"
 	"path/filepath"
+	"regexp"
 	"strings"
 	"sync"
 	"unicode"
@@ -83,25 +84,21 @@
 }
 
 var tags = []struct{ version, buildTags string }{
-	{"10.0.0", "go1.10"},
-	{"", "!go1.10"},
+	{"9.0.0", "!go1.10"},
+	{"10.0.0", "go1.10, !go1.13"},
+	{"11.0.0", "go1.13"},
 }
 
 // buildTags reports the build tags used for the current Unicode version.
 func buildTags() string {
 	v := UnicodeVersion()
-	for _, x := range tags {
-		// We should do a numeric comparison, but including the collate package
-		// would create an import cycle. We approximate it by assuming that
-		// longer version strings are later.
-		if len(x.version) <= len(v) {
-			return x.buildTags
-		}
-		if len(x.version) == len(v) && x.version <= v {
-			return x.buildTags
+	for _, e := range tags {
+		if e.version == v {
+			return e.buildTags
 		}
 	}
-	return tags[0].buildTags
+	log.Fatalf("Unknown build tags for Unicode version %q.", v)
+	return ""
 }
 
 // IsLocal reports whether data files are available locally.
@@ -269,12 +266,29 @@
 	}
 }
 
-func insertVersion(filename, version string) string {
+func fileToPattern(filename string) string {
 	suffix := ".go"
 	if strings.HasSuffix(filename, "_test.go") {
 		suffix = "_test.go"
 	}
-	return fmt.Sprint(filename[:len(filename)-len(suffix)], version, suffix)
+	prefix := filename[:len(filename)-len(suffix)]
+	return fmt.Sprint(prefix, "%s", suffix)
+}
+
+func updateBuildTags(pattern string) {
+	for _, t := range tags {
+		oldFile := fmt.Sprintf(pattern, t.version)
+		b, err := ioutil.ReadFile(oldFile)
+		if err != nil {
+			continue
+		}
+		build := fmt.Sprintf("// +build %s", t.buildTags)
+		b = regexp.MustCompile(`// \+build .*`).ReplaceAll(b, []byte(build))
+		err = ioutil.WriteFile(oldFile, b, 0644)
+		if err != nil {
+			log.Fatal(err)
+		}
+	}
 }
 
 // WriteVersionedGoFile prepends a standard file comment, adds build tags to
@@ -282,16 +296,16 @@
 // the given bytes, applies gofmt, and writes them to a file with the given
 // name. It will call log.Fatal if there are any errors.
 func WriteVersionedGoFile(filename, pkg string, b []byte) {
-	tags := buildTags()
-	if tags != "" {
-		filename = insertVersion(filename, UnicodeVersion())
-	}
+	pattern := fileToPattern(filename)
+	updateBuildTags(pattern)
+	filename = fmt.Sprintf(pattern, UnicodeVersion())
+
 	w, err := os.Create(filename)
 	if err != nil {
 		log.Fatalf("Could not create file %s: %v", filename, err)
 	}
 	defer w.Close()
-	if _, err = WriteGo(w, pkg, tags, b); err != nil {
+	if _, err = WriteGo(w, pkg, buildTags(), b); err != nil {
 		log.Fatalf("Error writing file %s: %v", filename, err)
 	}
 }