hitting spacebar always adds a parameter to the first ancestor that is a CallBlock

src/eval.ts

@@ -470,4 +470,56 @@ function makeError(env: Environment, e: Error, unification: Unification=new Map(
     nextFreeTypeVar: idx + 1,
     typeVars: new Set([...env.typeVars, UNBOUND_SYMBOLS[idx]]),
   }];
-}
+}
+
+export function removeFocus(state: ExprBlockState): ExprBlockState {
+  if (state.kind === "input") {
+    return { ...state, focus: false };
+  }
+  else if (state.kind === "call") {
+    return {
+      ...state,
+      fn: removeFocus(state.fn),
+      input: removeFocus(state.input),
+    };
+  }
+  else if (state.kind === "lambda") {
+    return {
+      ...state,
+      focus: false,
+      expr: removeFocus(state.expr),
+    };
+  }
+  else { // state.kind === "let"
+    return {
+      ...state,
+      focus: false,
+      value: removeFocus(state.value),
+      inner: removeFocus(state.inner),
+    }
+  }
+}
+
+export function addFocusRightMost(state: ExprBlockState) : ExprBlockState {
+  if (state.kind === "input") {
+    return { ...state, focus: true };
+  }
+  else if (state.kind === "call") {
+    return {
+      ... state,
+      input: addFocusRightMost(state.input),
+    };
+  }
+  else if (state.kind === "lambda") {
+    return {
+      ...state,
+      expr: addFocusRightMost(state.expr),
+    };
+  }
+  else { // state.kind === "let"
+    return {
+      ...state,
+      inner: addFocusRightMost(state.inner),
+    }
+  }
+}