A reducer is used for reducing a set of elements.
A reducer is represented as a tuple of the three functions: 1) initialization function: key => initial state 2) reduce function: (old state, input) => new state 3) finalization function: state => result
__init__( init_func, reduce_func, finalize_func )
THIS FUNCTION IS DEPRECATED. It will be removed in a future version.
Instructions for updating: