diff --git a/README.md b/README.md
index 84f0bd8..4f44d1b 100644
--- a/README.md
+++ b/README.md
@@ -1101,6 +1101,8 @@ class MySortable:
 ```
 
 ### Iterator
+* **Next() should return next item or raise 'StopIteration'.**
+* **Iter() should return 'self'.**
 ```python
 class Counter:
     def __init__(self):
@@ -1205,7 +1207,7 @@ class MyCollection:
 ```
 
 ### Sequence
-* **Only required methods are len() and getitem().**
+* **Only required methods are len() and getitem(), that should return an item at index or raise 'IndexError'.**
 * **Iter() and contains() automatically work on any object that has getitem() defined.**
 * **Reversed() automatically works on any object that has getitem() and len() defined.**
 ```python
diff --git a/index.html b/index.html
index e260b27..bb91ed6 100644
--- a/index.html
+++ b/index.html
@@ -1034,6 +1034,10 @@ Z = dataclasses.make_dataclass(<span class="hljs-string">'Z'</span>, [<span clas
         <span class="hljs-keyword">return</span> <span class="hljs-built_in">NotImplemented</span>
 </code></pre>
 <h3 id="iterator-1">Iterator</h3>
+<ul>
+<li><strong>Next() should return next item or raise 'StopIteration'.</strong></li>
+<li><strong>Iter() should return 'self'.</strong></li>
+</ul>
 <pre><code class="python language-python hljs"><span class="hljs-class"><span class="hljs-keyword">class</span> <span class="hljs-title">Counter</span>:</span>
     <span class="hljs-function"><span class="hljs-keyword">def</span> <span class="hljs-title">__init__</span><span class="hljs-params">(self)</span>:</span>
         self.i = <span class="hljs-number">0</span>
@@ -1120,7 +1124,7 @@ lock = threading.RLock(); <span class="hljs-keyword">with</span> lock: ...
 </code></pre>
 <h3 id="sequence">Sequence</h3>
 <ul>
-<li><strong>Only required methods are len() and getitem().</strong></li>
+<li><strong>Only required methods are len() and getitem(), that should return an item at index or raise 'IndexError'.</strong></li>
 <li><strong>Iter() and contains() automatically work on any object that has getitem() defined.</strong></li>
 <li><strong>Reversed() automatically works on any object that has getitem() and len() defined.</strong></li>
 </ul>