diff --git a/src/array.ts b/src/array.ts
index 89091d1..c1dfd15 100644
--- a/src/array.ts
+++ b/src/array.ts
@@ -273,7 +273,7 @@ export const mapFlat = <T, NF extends boolean = false>(
   nf?: NF
 ) => {
   const coll = collector<MapCell<T[], NF>>(proxy);
-  return proxy.map(
+  return proxy.mapNoPrevious(
     [arr],
     (cells) => coll(proxy.mapNoPrevious(cells, (..._cells) => _cells)),
     name,
diff --git a/src/cellify.test.ts b/src/cellify.test.ts
index 3d638f5..65d099b 100644
--- a/src/cellify.test.ts
+++ b/src/cellify.test.ts
@@ -12,7 +12,7 @@ import { Sheet } from "./sheet";
 
 type IsEqual<T, U> = [T] extends [U] ? ([U] extends [T] ? true : false) : false;
 
-test("", () => {
+test("IsEqual type", () => {
   type T = { a: string[] }[];
   type C = Cellified<T>;
   type U = Uncellified<C>;
@@ -83,3 +83,19 @@ test("cellify failOnCell", async () => {
   const v = { a: [1, 2, 3], b: { c: { foo: proxy.new(1, "1"), bar: 1 } } };
   expect(() => cellify(proxy, v, "cv", true)).toThrowError("value is cell");
 });
+
+test("cellify failOnError", async () => {
+  const sheet = new Sheet();
+  const proxy = new SheetProxy(sheet);
+  const v = proxy.new(1);
+  // @ts-expect-error intentional
+  const m = v.map((v) => v.toLowerCase());
+  // The standard uncellify call throws.
+  await expect(() => uncellify(m)).rejects.toThrow(
+    "toLowerCase is not a function"
+  );
+  // But we retrieve the error with errorsAsValues.
+  await expect(uncellify(m, { errorsAsValues: true })).resolves.toBeInstanceOf(
+    Error
+  );
+});
diff --git a/src/cellify.ts b/src/cellify.ts
index 83eded4..67671fd 100644
--- a/src/cellify.ts
+++ b/src/cellify.ts
@@ -1,4 +1,11 @@
-import { type AnyCell, Cell, type MapCell, type ValueCell } from "./cell";
+import {
+  type AnyCell,
+  Cell,
+  type CellResult,
+  type MapCell,
+  type Pending,
+  type ValueCell
+} from "./cell";
 import { collector } from "./gc";
 import type { SheetProxy } from "./proxy";
 
@@ -22,14 +29,15 @@ export type Uncellified<T> = T extends AnyCell<infer U>
     : U
   : T;
 
-// @todo is type only if true
-// exclude classes
+// isObject returns true if the value is a regular JavaScript Object,
+// but not null neither a custom Class instance.
 export const isObject = <K extends string | number | symbol>(
   v: unknown
 ): v is Record<K, unknown> =>
   typeof v === "object" && v !== null && v.constructor?.name === "Object";
 
 const errIsCell = new Error("value is cell");
+
 /**
  * cellify converts any value to a Cellified value where each array or record
  * becomes a Cell in canonical form.
@@ -63,24 +71,39 @@ export const cellify = <T>(
   ) as Cellified<T>;
 };
 
+export type UncellifyOptions = {
+  getter?: <T>(c: AnyCell<T>) => Pending<T, boolean> | CellResult<T, boolean>;
+  errorsAsValues?: boolean;
+};
+
 /**
  * uncellify is used in tests to flatten a value tree that contains multiple cells.
  * @param v any value
  * @returns value without cells
  */
 export const uncellify = async <T>(
-  v: T | AnyCell<T>
+  v: T | AnyCell<T>,
+  options: UncellifyOptions = {}
 ): Promise<Uncellified<T>> => {
-  const value = v instanceof Cell ? await v.consolidatedValue : v;
-  if (value instanceof Error) throw value;
+  const getter = options?.getter
+    ? options.getter
+    : (cell: AnyCell<unknown>) => cell.consolidatedValue;
+  const value = v instanceof Cell ? await getter(v) : v;
+  if (value instanceof Error) {
+    if (options?.errorsAsValues) return value as Uncellified<T>;
+    throw value;
+  }
   if (Array.isArray(value))
-    return Promise.all(value.map((_element) => uncellify(_element))) as Promise<
-      Uncellified<T>
-    >;
+    return Promise.all(
+      value.map((_element) => uncellify(_element, options))
+    ) as Promise<Uncellified<T>>;
   if (isObject(value))
     return Object.fromEntries(
       await Promise.all(
-        Object.entries(value).map(async ([k, vv]) => [k, await uncellify(vv)])
+        Object.entries(value).map(async ([k, vv]) => [
+          k,
+          await uncellify(vv, options)
+        ])
       )
     );
   // Classes, null or base types (string, number, ...)
diff --git a/src/debouncer.ts b/src/debouncer.ts
new file mode 100644
index 0000000..a35553c
--- /dev/null
+++ b/src/debouncer.ts
@@ -0,0 +1,30 @@
+import type { ValueCell } from "./cell";
+
+export type Debouncer = <T>(cb: (v: T) => void | Promise<void>, v: T) => void;
+
+/**
+ * debouncer creates a debounce function that will execute a callback after a _delay_.
+ *
+ * Create with `const debounce = debouncer()`
+ * and use as `debounce(cb, v, delay)`.
+ * @param cb callback
+ * @param v value passed to callback
+ * @param delay optional delay in ms, default: 750
+ */
+export const debouncer = (
+  delay = 750,
+  working: ValueCell<boolean> | undefined = undefined
+): Debouncer => {
+  // console.log({ setting: delay });
+  let timer: ReturnType<typeof setTimeout>;
+  return <T>(cb: (v: T) => void | Promise<void>, v: T) => {
+    // console.log({ called: delay });
+    if (working !== undefined) working.set(true);
+    clearTimeout(timer);
+    timer = setTimeout(async () => {
+      // console.log({ deb: delay });
+      await cb(v);
+      if (working !== undefined) working.set(false);
+    }, delay);
+  };
+};
diff --git a/src/index.ts b/src/index.ts
index f0d515b..9409b49 100644
--- a/src/index.ts
+++ b/src/index.ts
@@ -52,11 +52,14 @@ export {
 } from "./cellify";
 export { clock, clockWork, type Clock } from "./clock";
 export { copy } from "./copy";
+export { debouncer, type Debouncer } from "./debouncer";
 export { Debugger } from "./debug";
+export { initialValue } from "./initial";
 export { jsonStringify } from "./json";
 export { nextSubscriber } from "./next";
 export {
   asyncReduce,
+  flattenObject,
   mapObject,
   reduceObject,
   type CellObject
diff --git a/src/initial.test.ts b/src/initial.test.ts
new file mode 100644
index 0000000..27992f3
--- /dev/null
+++ b/src/initial.test.ts
@@ -0,0 +1,16 @@
+import { expect, test } from "vitest";
+
+import { initialValue } from "./initial";
+import { isEqual } from "./isEqual.test";
+import { delayed, sleep } from "./promise";
+import { Sheet } from "./sheet";
+
+test("initialValue", async () => {
+  const proxy = new Sheet(isEqual).newProxy();
+  const a = proxy.new(1);
+  const b = proxy.new(delayed(2, 50));
+  const c = initialValue(proxy, a, b);
+  expect(c.consolidatedValue).toBe(1);
+  await sleep(60);
+  expect(c.consolidatedValue).toBe(2);
+});
diff --git a/src/initial.ts b/src/initial.ts
new file mode 100644
index 0000000..457be71
--- /dev/null
+++ b/src/initial.ts
@@ -0,0 +1,19 @@
+import type { AnyCell, MapCell } from "./cell";
+import type { SheetProxy } from "./proxy";
+
+export const initialValue = <T>(
+  proxy: SheetProxy,
+  v0: T | AnyCell<T>,
+  v: AnyCell<T>,
+  name = "initial"
+): MapCell<T, true> => {
+  const cell = proxy.new(v0, name);
+  v.subscribe((v) => {
+    // We do not propagate errors yet.
+    if (v instanceof Error) return;
+    cell.set(v);
+  });
+  // We fake being a MapCell to prevent setting the cell
+  // outside of this function.
+  return cell as unknown as MapCell<T, true>;
+};
diff --git a/src/json.ts b/src/json.ts
index de88b88..e03e147 100644
--- a/src/json.ts
+++ b/src/json.ts
@@ -57,7 +57,7 @@ export const jsonStringify = <T>(
       case "symbol":
         break;
       case "bigint":
-        out += v < 1_000_000n ? Number(v) : `"${v.toString()}"`;
+        out += v < 1_000_000_000n ? Number(v) : `"${v.toString()}"`;
         break;
       default:
         out += JSON.stringify(v);
diff --git a/src/object.test.ts b/src/object.test.ts
index 6050607..09bc4d6 100644
--- a/src/object.test.ts
+++ b/src/object.test.ts
@@ -4,13 +4,13 @@ import { expect, test } from "vitest";
 import { cellify, uncellify } from "./cellify";
 import { Debugger } from "./debug";
 import { isEqual } from "./isEqual.test";
-import { asyncReduce, mapObject, reduceObject } from "./object";
+import { asyncReduce, flattenObject, mapObject, reduceObject } from "./object";
 import { delayed } from "./promise";
 import { SheetProxy } from "./proxy";
 import { Sheet } from "./sheet";
 
 test("mapObject", async () => {
-  const sheet = new Sheet();
+  const sheet = new Sheet(isEqual);
   const proxy = new SheetProxy(sheet);
 
   const obj = cellify(proxy, { a: 1, b: "foo", c: "bar" });
@@ -60,7 +60,7 @@ test("asyncReduce", async () => {
 test(
   "reduceObject",
   async () => {
-    const sheet = new Sheet();
+    const sheet = new Sheet(isEqual);
     const debug = new Debugger(sheet);
     const proxy = new SheetProxy(sheet);
 
@@ -89,3 +89,18 @@ test(
   },
   { timeout: 1000 }
 );
+
+test("flattenObject", async () => {
+  const sheet = new Sheet(isEqual);
+  const debug = new Debugger(sheet);
+  const proxy = new SheetProxy(sheet);
+
+  const obj = cellify(proxy, { a: 1, b: 2, c: 3 });
+  const f = flattenObject(proxy, obj);
+  await expect(f.consolidatedValue).resolves.toEqual({ a: 1, b: 2, c: 3 });
+  expect(sheet.stats).toEqual({ count: 6, size: 6 }); // 3+1 obj +1 flatten +1 pointer
+
+  await (await obj.consolidatedValue).a.set(4);
+  await expect(f.get()).resolves.toEqual({ a: 4, b: 2, c: 3 });
+  expect(sheet.stats).toEqual({ count: 6, size: 6 }); // unchanged
+});
diff --git a/src/object.ts b/src/object.ts
index 3d9df55..cb2f8e8 100644
--- a/src/object.ts
+++ b/src/object.ts
@@ -11,7 +11,6 @@ export type CellObject<T> = AnyCell<Record<string, AnyCell<T>>>;
 export const mapObject = <T, U, NF extends boolean = false>(
   proxy: SheetProxy,
   obj: CellObject<T>,
-  // @todo return type
   fn: (
     key: string,
     value: T,
@@ -22,7 +21,7 @@ export const mapObject = <T, U, NF extends boolean = false>(
 ): MapCell<Record<string, AnyCell<U>>, NF> =>
   proxy.map(
     [obj],
-    (cells, prev) => {
+    async (cells, prev) => {
       const set = new Set(Object.keys(prev || {}));
       const res = Object.fromEntries(
         Object.entries(cells).map(([k, v]) => {
@@ -83,7 +82,7 @@ export const reduceObject = <T, R, NF extends boolean = false>(
   const coll = collector<MapCell<R, NF>>(proxy);
   return proxy.mapNoPrevious(
     [obj],
-    (cells) => {
+    async (cells) => {
       const keys = Object.keys(cells);
       const values = Object.values(cells);
       // console.log({ reduce: keys, name, count: proxy._sheet.stats.count });
@@ -104,3 +103,26 @@ export const reduceObject = <T, R, NF extends boolean = false>(
     nf
   );
 };
+
+export const flattenObject = <T, NF extends boolean = false>(
+  proxy: SheetProxy,
+  obj: CellObject<T>,
+  name = "flatten",
+  nf?: NF
+) => {
+  const coll = collector<MapCell<Record<string, T>, NF>>(proxy);
+  return proxy.mapNoPrevious(
+    [obj],
+    async (cells) => {
+      const keys = Object.keys(cells);
+      const values = Object.values(cells);
+      return coll(
+        proxy.mapNoPrevious(values, (..._cells) =>
+          Object.fromEntries(_cells.map((v, i) => [keys[i], v]))
+        )
+      );
+    },
+    name,
+    nf
+  );
+};
diff --git a/src/proxy.test.ts b/src/proxy.test.ts
index 7523e49..3357f4b 100644
--- a/src/proxy.test.ts
+++ b/src/proxy.test.ts
@@ -29,23 +29,26 @@ test("native proxy", () => {
   expect(trigger).toBeTruthy();
 });
 
-test("SheetProxy", () => {
-  const store = new Sheet();
-  const value = store.new(2);
-  const proxy = new SheetProxy(store);
+test("SheetProxy", async () => {
+  const sheet = new Sheet();
+  const value = sheet.new(2);
+  const proxy = new SheetProxy(sheet);
   const double = proxy.map([value], (x) => 2 * x);
   const add = proxy.map([double], (x) => x + 1);
   proxy.destroy();
+  // Force GC collection
+  sheet.collection();
+  await proxy.working.wait();
   value.set(3); // will not update detached/deleted cells
   expect(double.value).toBe(4);
   expect(add.value).toBe(5);
 });
 
 test("Sheet multiple async updates", async () => {
-  const store = new Sheet();
-  const value = store.new(2);
+  const sheet = new Sheet();
+  const value = sheet.new(2);
   const double = value.map(async (x) => delayed(2 * x, 50));
-  const add = store.map([value, double], (value, double) =>
+  const add = sheet.map([value, double], (value, double) =>
     delayed(value + double + 1, 30)
   );
   // console.log("value", value.id, "double", double.id, "add", add.id);
@@ -54,7 +57,7 @@ test("Sheet multiple async updates", async () => {
   value.set(3);
   expect(await add.consolidatedValue).toBe(10);
   value.set(4);
-  await store.wait();
+  await sheet.wait();
   expect(await add.consolidatedValue).toBe(13);
 });
 
@@ -98,9 +101,13 @@ test("proxy deletion", async () => {
   const c = sub.map([a, b], async (a, b) => a + b);
   expect(sheet.stats).toEqual({ count: 3, size: 3 });
   sub.destroy();
+  await proxy.working.wait();
   expect(sheet.stats).toEqual({ count: 3, size: 2 });
   await expect(b.get()).resolves.toBe(2);
   proxy.destroy();
+  // Force collection.
+  sheet.collection();
+  // Collection is not happening yet.
   expect(sheet.stats).toEqual({ count: 3, size: 0 });
 });
 
@@ -113,5 +120,7 @@ test("proxy deletion with loop", async () => {
   const c = sub.map([a, b], async (a, b) => delayed(a + b, 5));
   const d = proxy.map([c], async (v) => delayed(v * 2, 15));
   expect(sheet.stats).toEqual({ count: 4, size: 4 });
-  expect(() => sub.destroy()).toThrow("Cell has references");
+  // Since we now collect the whole subgraph, there is no error.
+  sub.destroy();
+  // expect(() => sub.destroy()).toThrow("Cell has references");
 });
diff --git a/src/proxy.ts b/src/proxy.ts
index 64ff258..f220d8c 100644
--- a/src/proxy.ts
+++ b/src/proxy.ts
@@ -167,6 +167,31 @@ export class SheetProxy {
     name?: string,
     noFail?: NF
   ): MapCell<V, NF>;
+  map<D1, D2, D3, D4, D5, D6, D7, D8, V, NF extends boolean = false>(
+    dependencies: [
+      AnyCell<D1>,
+      AnyCell<D2>,
+      AnyCell<D3>,
+      AnyCell<D4>,
+      AnyCell<D5>,
+      AnyCell<D6>,
+      AnyCell<D7>,
+      AnyCell<D8>
+    ],
+    computeFn: (
+      arg1: D1,
+      arg2: D2,
+      arg3: D3,
+      arg4: D4,
+      arg5: D5,
+      arg6: D6,
+      arg7: D7,
+      arg8: D8,
+      prev?: V
+    ) => V | Promise<V | AnyCell<V>> | AnyCell<V>,
+    name?: string,
+    noFail?: NF
+  ): MapCell<V, NF>;
 
   /**
    * map a list to cells to a new cells using the compute function.
@@ -250,13 +275,9 @@ export class SheetProxy {
 
   /**
    * destroy the Proxy and free memory.
-   * @todo check for memory leaks
    */
   destroy() {
-    // if (!this?._sheet) {
-    //   throw new Error(`missing: ${this?._name}`);
-    // }
-    this._sheet.delete(...this._list);
+    this._sheet.collect(...this._list);
     this._list = [];
   }
 }
diff --git a/src/sheet.ts b/src/sheet.ts
index 39fa50b..e7f6906 100644
--- a/src/sheet.ts
+++ b/src/sheet.ts
@@ -567,12 +567,7 @@ export class Sheet {
         this._internalNotify(_result.done);
 
         // Collect garbage
-        if (this._gc.size) {
-          const l = Array.from(this._gc);
-          this.debug(l, "gc", { deleting: l });
-          this._gc = new Set();
-          this.delete(...l);
-        }
+        this.collection();
 
         // End of the update
         release();
@@ -592,6 +587,15 @@ export class Sheet {
     );
   }
 
+  collection() {
+    if (this._gc.size) {
+      const l = Array.from(this._gc);
+      this.debug(l, "gc", { deleting: l });
+      this._gc = new Set();
+      this.delete(...l);
+    }
+  }
+
   private registerCancelAndDone<V>(
     updatable: number[],
     computations: (V | Canceled | Error)[],
@@ -957,7 +961,7 @@ export class Sheet {
     this.debug(ids, "collect", { collecting: ids, cells: input });
     for (const id of ids) {
       const deps = this.g.partialTopologicalSort(id);
-      this.debug(undefined, "collect", { deps });
+      // this.debug(undefined, "collect", { deps });
       for (const dep of deps) this._gc.add(dep);
     }
   }