code: @Override
  	public void combine(Iterator<Record> records, Collector<Record> out) throws Exception {
  		// to be implemented, if the reducer should use a combiner. Note that the combining method
  		// is only used, if the stub class is further annotated with the annotation
  		// @ReduceOperator.Combinable
  		reduce(records, out);
  	}
docs: No default implementation provided.
  This method must be overridden by reduce stubs that want to make use of the combining feature.
  In addition, the ReduceFunction extending class must be annotated as Combinable.
  Note that this function must be implemented, if the reducer is annotated as combinable.
  <p>
  The use of the combiner is typically a pre-reduction of the data. It works similar as the reducer, only that is
  is not guaranteed to see all values with the same key in one call to the combine function. Since it is called
  prior to the <code>reduce()</code> method, input and output types of the combine method are the input types of
  the <code>reduce()</code> method.
  
  @see eu.stratosphere.api.java.record.operators.ReduceOperator.Combinable
  @param records
  The records to be combined. Unlike in the reduce method, these are not necessarily all records
  belonging to the given key.
  @param out The collector to write the result to.
  
  @throws Exception Implementations may forward exceptions, which are caught by the runtime. When the
  runtime catches an exception, it aborts the combine task and lets the fail-over logic
  decide whether to retry the combiner execution.