support some filtering

internal/nature.go

@@ -15,8 +15,17 @@ const (
 )
 
 func (n Nature) String() string {
-	if n == "" {
-		return "unknown"
+	if n.Valid() {
+		return string(n)
+	}
+	return "unknown"
+}
+
+func (n Nature) Valid() bool {
+	switch n {
+	case NatureG01, NatureG20:
+		return true
+	default:
+		return false
 	}
-	return string(n)
 }