@abstractmethod<EOL><INDENT>def reduce(self, context):<DEDENT>
pass<EOL>
Called once for each key. Applications must override this, emitting
an output key/value pair through the context.

:type context: :class:`Context`
:param context: the context object passed by
  the framework, used to get the input key and corresponding
  set of values and emit the output key/value pair.