code: public FaunusPipeline groupCount(final String keyClosure, final String valueClosure) {
          this.state.assertNotLocked();
  
  
          this.compiler.addMapReduce(GroupCountMapReduce.Map.class,
                  GroupCountMapReduce.Combiner.class,
                  GroupCountMapReduce.Reduce.class,
                  Text.class,
                  LongWritable.class,
                  Text.class,
                  LongWritable.class,
                  GroupCountMapReduce.createConfiguration(this.state.getElementType(),
                          this.validateClosure(keyClosure), this.validateClosure(valueClosure)));
  
          makeMapReduceString(GroupCountMapReduce.class);
          return this;
      }
docs: Apply the provided closure to the incoming element to determine the grouping key.
  Then apply the value closure to the current element to determine the count increment.
  The results are stored in the jobs sideeffect file in HDFS.
  
  @return the extended FaunusPipeline.