Update syntax.md

docs/_pages/syntax.md

@@ -207,6 +207,15 @@ for k, v in {1, 4, 9} do
 end
 ```
 
-This works for tables but can also be extended for tables or userdata by implementing `__iter` metamethod that is called before the iteration begins, and should return an iterator function like `next` (or a custom one).
+This works for tables but can also be extended for tables or userdata by implementing `__iter` metamethod that is called before the iteration begins, and should return an iterator function like `next` (or a custom one):
 
-For tables, the iteration order is specified to be consecutive for elements `1..#t` and unordered after that, visiting every element; similarly to iteration using `pairs`, modifying the table entries for keys other than the current one results in unspecified behavior.
+```lua
+local obj = { items = {1, 4, 9} }
+setmetatable(obj, { __iter = function(o) return next, o.items end })
+
+for k, v in obj do
+    assert(k * k == v)
+end
+```
+
+The default iteration order for tables is specified to be consecutive for elements `1..#t` and unordered after that, visiting every element; similarly to iteration using `pairs`, modifying the table entries for keys other than the current one results in unspecified behavior.