diff --git a/examples/logic/eval b/examples/logic/eval
index dc41896..8884a28 100755
--- a/examples/logic/eval
+++ b/examples/logic/eval
@@ -6,8 +6,10 @@ import json as _json
 
 def evaluate(node):
 	# _sys.stderr.write("eval: %s\n" % _json.dumps(node))
-	if (node["label"] == "constant"):
-		return node["value"][0]["data"]["value"]
+	if (node["label"] == "constant_false"):
+		return False
+	elif (node["label"] == "constant_true"):
+		return True
 	elif (node["label"] == "negation"):
 		return (not evaluate(node["children"][0]))
 	elif (node["label"] == "conjunction"):
diff --git a/examples/logic/logic.tp2.json b/examples/logic/logic.tp2.json
index ddb8af4..745d6f8 100644
--- a/examples/logic/logic.tp2.json
+++ b/examples/logic/logic.tp2.json
@@ -1,4 +1,5 @@
 {
+	"version": "1",
 	"lexer_rules": [
 		{
 			"type": "ignore",
@@ -7,19 +8,17 @@
 			}
 		},
 		{
-			"type": "boolean",
+			"type": "string",
 			"parameters": {
 				"pattern": "false|0",
-				"id": "const",
-				"value": false
+				"id": "const_false"
 			}
 		},
 		{
-			"type": "boolean",
+			"type": "string",
 			"parameters": {
 				"pattern": "true|1",
-				"id": "const",
-				"value": true
+				"id": "const_true"
 			}
 		},
 		{
@@ -60,10 +59,17 @@
 	],
 	"parser_rules": [
 		{
-			"label": "constant",
+			"label": "constant_false",
 			"premise": "formula",
 			"conclusion": [
-				{"type": "terminal", "parameters": {"id": "const"}}
+				{"type": "terminal", "parameters": {"id": "const_false"}}
+			]
+		},
+		{
+			"label": "constant_true",
+			"premise": "formula",
+			"conclusion": [
+				{"type": "terminal", "parameters": {"id": "const_true"}}
 			]
 		},
 		{