Behavior specs cleanup: Improve label specs

spec/behavior/generated-parser-behavior.spec.js

@@ -724,13 +724,22 @@ describe("generated parser behavior", function() {
     });
 
     describe("label", function() {
-      it("delegates to the expression", function() {
+      describe("when the expression matches", function() {
+        it("returns its match result", function() {
           var parser = PEG.buildParser('start = a:"a"', options);
 
           expect(parser).toParse("a", "a");
+        });
+      });
+
+      describe("when the expression doesn't match", function() {
+        it("reports match failure", function() {
+          var parser = PEG.buildParser('start = a:"a"', options);
+
           expect(parser).toFailToParse("b");
         });
       });
+    });
 
     describe("sequence", function() {
       it("matches correctly", function() {