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 <details open><summary><strong>What exactly is <code>&lt;collection&gt;</code>?</strong></summary><br>
 &nbsp;&nbsp;&nbsp;&nbsp;Collection is my name for an iterable object. An iterable object in Python is any object that has at least one of iter() and getitem() special methods defined. By convention, <code>&lt;object&gt;.__iter__()</code> should return an iterator of object's items and <code>&lt;object&gt;.__getitem__(&lt;index&gt;)</code> an item at that index. I chose not to use the name iterable because it sounds scarier and more vague than collection, even though it has a precise definition.<br><br>
-&nbsp;&nbsp;&nbsp;&nbsp;To make matters a bit more confusing, an abstract base class called Iterable doesn't fully follow this definition. An expression <code>isinstance(&lt;object&gt;, collections.abc.Iterable)</code> only checks whether an object has iter() special method, disregarding the getitem().<br><br>
-&nbsp;&nbsp;&nbsp;&nbsp;Although collection has no definition in Python's <a href="https://docs.python.org/3/glossary.html">glossary</a>, there exists a Collection abstract base class. Expression <code>isinstance(&lt;object&gt;, collections.abc.Collection)</code> returns 'True' for any object that has len() and at least one of iter() and getitem() special methods defined. By convention, <code>&lt;object&gt;.__len__()</code> should return the number of object's elements.
+&nbsp;&nbsp;&nbsp;&nbsp;To make matters a bit more confusing, an abstract base class called Iterable doesn't fully follow this definition. An expression <code>instanceof(&lt;object&gt;, collections.abc.Iterable)</code> only checks whether an object has iter() special method, disregarding the getitem().<br><br>
+&nbsp;&nbsp;&nbsp;&nbsp;Although collection has no definition in Python's <a href="https://docs.python.org/3/glossary.html">glossary</a>, there exists a Collection abstract base class. Expression <code>instanceof(&lt;object&gt;, collections.abc.Collection)</code> returns 'True' for any object that has len(), iter() and contains() special methods defined. <code>&lt;object&gt;.__len__()</code> should return the number of elements and <code>&lt;object&gt;.__contains__(&lt;el&gt;)</code> should check if object contains the passed element.
 </details><br>
 
 <details open><summary><strong>What about PEP 8?</strong></summary><br>