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Reducers
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A reducer is a function that takes a stream of data and combines it into a 
single value. It does this by processing each item one by one and updating an 
accumulated result.

For example, a reducer can:

* Sum a list of numbers.
* Count items in a stream.
* Build an object or array from incoming values.

.. toctree::
   :maxdepth: 1
   :hidden:

   list/index
   extend