diff --git a/benchmarks/fib.ce b/benchmarks/fib.ce
new file mode 100644
index 00000000..28d68daf
--- /dev/null
+++ b/benchmarks/fib.ce
@@ -0,0 +1,16 @@
+var time = use('time')
+
+function fib(n) {
+  if (n<2) return n
+  return fib(n-1) + fib(n-2)
+}
+
+var now = time.number()
+var arr = [1,2,3,4,5]
+for (var i in arr) {
+  log.console(fib(28))
+}
+
+log.console(`elapsed: ${time.number()-now}`)
+
+$_.stop()
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diff --git a/docs/cell.md b/docs/cell.md
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index 00000000..bdc8c9c4
--- /dev/null
+++ b/docs/cell.md
@@ -0,0 +1,180 @@
+# Cell Language
+
+Cell is a JavaScript variant used in the Prosperon game engine. While very similar to JavaScript, it has several important differences that make it more suitable for game development and actor-based programming.
+
+## Key Differences from JavaScript
+
+### Null vs Undefined
+- Cell has only `null`, no `undefined`
+- Idiomatic null checking: `if (object.x == null)`
+- Uninitialized variables and missing properties return `null`
+
+### Equality Operators
+- Only `==` operator exists (no `===`)
+- `==` is always strict (no type coercion)
+- `!=` for inequality (no `!==`)
+
+### Variable Declarations
+- `def` keyword for constants (replaces `const`)
+- `var` works like `let` (block-scoped)
+- No `let` keyword
+
+### Compilation
+- All code is compiled in strict mode
+- No need for `"use strict"` directive
+
+### Removed Features
+Cell removes several JavaScript features for simplicity and security:
+- No `Proxy` objects
+- No ES6 module syntax (use `use()` function instead)
+- No `class` syntax (use prototypes and closures)
+- No `Reflect` API
+- No `BigInt`
+- No `WeakMap`, `WeakSet`, `WeakRef`
+- No `document.all` (HTMLAllCollection)
+- No `with` statement
+- No `Date` intrinsic (use `time` module instead)
+
+## Language Features
+
+### Constants
+```javascript
+def PI = 3.14159
+def MAX_PLAYERS = 4
+// PI = 3.14 // Error: cannot reassign constant
+```
+
+### Variables
+```javascript
+var x = 10
+{
+  var y = 20  // Block-scoped like let
+  x = 15      // Can access outer scope
+}
+// y is not accessible here
+```
+
+### Null Checking
+```javascript
+var obj = {name: "player"}
+if (obj.score == null) {
+  obj.score = 0
+}
+```
+
+### Functions
+```javascript
+// Function declaration
+function add(a, b) {
+  return a + b
+}
+
+// Function expression
+var multiply = function(a, b) {
+  return a * b
+}
+
+// Arrow functions work normally
+var square = x => x * x
+```
+
+### Objects and Prototypes
+```javascript
+// Object creation
+var player = {
+  x: 0,
+  y: 0,
+  move: function(dx, dy) {
+    this.x += dx
+    this.y += dy
+  }
+}
+
+// Prototype-based inheritance
+function Enemy(x, y) {
+  this.x = x
+  this.y = y
+}
+Enemy.prototype.attack = function() {
+  // Attack logic
+}
+```
+
+### Module System
+Cell uses a custom module system with the `use()` function:
+```javascript
+var math = use('math')
+var draw2d = use('prosperon/draw2d')
+```
+
+### Time Handling
+Since there's no `Date` object, use the `time` module:
+```javascript
+var time = use('time')
+var now = time.number()     // Numeric timestamp
+var record = time.record()   // Structured time
+var text = time.text()       // Human-readable time
+```
+
+## Best Practices
+
+1. **Prefer `def` for values that won't change**
+   ```javascript
+   def TILE_SIZE = 32
+   var playerPos = {x: 0, y: 0}
+   ```
+
+2. **Always check for null explicitly**
+   ```javascript
+   if (player.weapon == null) {
+     player.weapon = createDefaultWeapon()
+   }
+   ```
+
+3. **Use prototype patterns instead of classes**
+   ```javascript
+   function GameObject(x, y) {
+     this.x = x
+     this.y = y
+   }
+   GameObject.prototype.update = function(dt) {
+     // Update logic
+   }
+   ```
+
+4. **Leverage closures for encapsulation**
+   ```javascript
+   function createCounter() {
+     var count = 0
+     return {
+       increment: function() { count++ },
+       getValue: function() { return count }
+     }
+   }
+   ```
+
+## Common Gotchas
+
+1. **No undefined means different behavior**
+   ```javascript
+   var obj = {}
+   console.log(obj.missing)  // null, not undefined
+   ```
+
+2. **Strict equality by default**
+   ```javascript
+   "5" == 5   // false (no coercion)
+   null == 0  // false
+   ```
+
+3. **Block-scoped var**
+   ```javascript
+   for (var i = 0; i < 10; i++) {
+     setTimeout(() => console.log(i), 100)  // Works as expected
+   }
+   ```
+
+## See Also
+- [Actor Model](actor.md) - Cell's actor-based programming model
+- [Module System](modules.md) - How to use and create modules
+- [API Reference](api/index.md) - Complete API documentation
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