def reduce_by_window(self, window_config, reduce_function):
from heronpy.streamlet.impl.reducebywindowbolt import ReduceByWindowStreamlet<EOL>reduce_streamlet = ReduceByWindowStreamlet(window_config, reduce_function, self)<EOL>self._add_child(reduce_streamlet)<EOL>return reduce_streamlet<EOL>
Return a new Streamlet in which each element of this Streamlet are collected
          over a window defined by window_config and then reduced using the reduce_function
          reduce_function takes two element at one time and reduces them to one element that
          is used in the subsequent operations.