diff --git a/rdf.proto b/rdf.proto
index 34f2b66..d05f458 100644
--- a/rdf.proto
+++ b/rdf.proto
@@ -12,16 +12,16 @@ message RdfIri {
 
 message RdfLiteral {
   string lex = 1;
-  // TODO: use literalKind.Empty instead of simple=true?
-  // Will it be smaller and/or faster?
   oneof literalKind {
-    bool simple = 2;
+    RdfLiteralSimple simple = 2;
     string langtag = 3;
     uint32 datatype = 4;
   }
 }
 
-// TODO: compare size & perf of empty message vs simple value (bool)
+message RdfLiteralSimple {
+}
+
 message RdfRepeat {
 }