use unused typevar when encountering unknown value or lambda parameter - type inferencing still not entirely correct

src/LambdaBlock.tsx

@@ -7,6 +7,7 @@ import { growEnv, TYPE_VARS } from "dope2";
 import { autoInputWidth } from "./util/dom_trickery";
 
 import "./LambdaBlock.css";
+import { getUnusedTypeVar } from "./eval";
 
 
 export interface LambdaBlockState {
@@ -31,12 +32,18 @@ export function LambdaBlock({state, setState, suggestionPriority}: LambdaBlockPr
     ...state,
     paramName,
   }));
-
   const setExpr = callback => setState(state => ({
     ...state,
     expr: callback(state.expr),
   }));
 
+  const onChangeName = (e) => {
+    if (state.paramName === "" && e.key === 'Backspace') {
+      setState(state => state.expr);
+      e.preventDefault();
+    }
+  };
+
   useEffect(() => {
     nameRef.current?.focus();
   }, []);
@@ -46,7 +53,7 @@ export function LambdaBlock({state, setState, suggestionPriority}: LambdaBlockPr
   const innerEnv = growEnv(env)(state.paramName)({
     kind: "unknown",
     i: undefined,
-    t: TYPE_VARS[0],
+    t: getUnusedTypeVar(env),
   });
   
   return <span>
@@ -58,13 +65,14 @@ export function LambdaBlock({state, setState, suggestionPriority}: LambdaBlockPr
           className='editable'
           value={state.paramName}
           placeholder="<name>"
+          onKeyDown={onChangeName}
           onChange={e => setParamName(e.target.value)}
           />
     </span>
     &nbsp;
     <span className="keyword">:</span>
     &nbsp;
-    <span className="lambdaExpr">
+    <div className="lambdaExpr">
       <EnvContext value={innerEnv}>
         <Editor
           state={state.expr}
@@ -75,6 +83,6 @@ export function LambdaBlock({state, setState, suggestionPriority}: LambdaBlockPr
           }}
           />
       </EnvContext>
-    </span>
+    </div>
   </span>
 }