add expression reducer

src/reducers/index.js

@@ -4,12 +4,14 @@ import thunk from "redux-thunk";
 
 import initialize from "./initialize";
 import cells from "./cells";
+import expression from "./expression";
 import controls from "./controls";
 import selectedMetadata from "./selectedMetadata";
 
 const Reducer = combineReducers({
   initialize,
   cells,
+  expression,
   controls,
   selectedMetadata,
 })