LLL: fix the set keyword (create symbol if not present)

test/liblll/EndToEndTest.cpp

@@ -57,6 +57,26 @@ BOOST_AUTO_TEST_CASE(panic)
 	BOOST_REQUIRE(m_output.empty());
 }
 
+BOOST_AUTO_TEST_CASE(variables)
+{
+	char const* sourceCode = R"(
+		(returnlll
+			(seq
+				(set 'x 1)
+				(set 'y 2)
+				;; this should equal to 3
+				(set 'z (add (get 'x) (get 'y)))
+				;; overwriting it here
+				(set 'y 4)
+				;; each variable has a 32 byte slot, starting from memory location 0x80
+				;; variable addresses can also be retrieved by x or (ref 'x)
+				(set 'k (add (add (ref 'x) (ref 'y)) z))
+				(return (add (add (get 'x) (add (get 'y) (get 'z))) (get 'k)))))
+	)";
+	compileAndRun(sourceCode);
+	BOOST_CHECK(callFallback() == encodeArgs(u256(488)));
+}
+
 BOOST_AUTO_TEST_CASE(when)
 {
 	char const* sourceCode = R"(