Correctly add agruments to positional list, added test case

Equation/core.py

@@ -714,7 +714,7 @@ def __compile(self):
                 __expect_op = False
             elif not __expect_op and v[1] == 'NAME':
                 self.__argsused.add(v[0])
-                if v not in self.__args:
+                if v[0] not in self.__args:
                     self.__args.append(v[0])
                 self.__expr.append(ExpressionVariable(v[0]))
                 __expect_op = True

tests/test_Equation.py

@@ -79,6 +79,19 @@ def testStr(self):
     def tearDown(self):
         pass
 
+class TestPostionalArgs(unittest.TestCase):
+    def setUp(self):
+        self.fn = Expression("x+x-y")
+
+    def testCall(self):
+        try:
+            self.fn(1,2)
+        except TypeError:
+            self.fail("Postional Args Rises a TypeError Exception.")
+
+    def tearDown(self):
+        pass
+
 class TestComplexEquation(unittest.TestCase):
     def setUp(self):
         pass