add explanatory comment

Python/compile.c

@@ -3769,6 +3769,8 @@ dict_keys_inorder(PyObject *dict, int offset)
 		return NULL;
 	while (PyDict_Next(dict, &pos, &k, &v)) {
 		i = PyInt_AS_LONG(v);
+		/* The keys of the dictionary are tuples. (see compiler_add_o)
+		   The object we want is always first, though. */
 		k = PyTuple_GET_ITEM(k, 0);
 		Py_INCREF(k);
 		assert((i - offset) < size);