Test whether tokens reference the same memory as runes

parse_test.go

@@ -121,6 +121,12 @@ func TestParseString (test *testing.T) {
 		logTokens(test, correctTokens)
 		test.FailNow()
 	}
-	// TODO: ensure runeLayout slices in the tokens reference the same
-	// memory as the complete runes slice
+	test.Logf("changing first rune from %c to x", runes[0].run)
+	runes[0].run = 'x'
+	tokenRune := tokens[0].runes[0].run
+	if tokenRune != 'x' {
+		test.Fatalf (
+			"tokens does not reference the same memory as runes after changing runes: %c, %c",
+			runes[0].run, tokenRune)
+	}
 }