code: public <R> SingleOutputStreamOperator<R> flatMap(
  			CoFlatMapFunction<IN1, IN2, R> coFlatMapper) {
  
  		TypeInformation<R> outTypeInfo = TypeExtractor.getBinaryOperatorReturnType(
  			coFlatMapper,
  			CoFlatMapFunction.class,
  			0,
  			1,
  			2,
  			TypeExtractor.NO_INDEX,
  			getType1(),
  			getType2(),
  			Utils.getCallLocationName(),
  			true);
  
  		return transform("Co-Flat Map", outTypeInfo, new CoStreamFlatMap<>(inputStream1.clean(coFlatMapper)));
  	}
docs: Applies a CoFlatMap transformation on a {@link ConnectedStreams} and
  maps the output to a common type. The transformation calls a
  {@link CoFlatMapFunction#flatMap1} for each element of the first input
  and {@link CoFlatMapFunction#flatMap2} for each element of the second
  input. Each CoFlatMapFunction call returns any number of elements
  including none.
  
  @param coFlatMapper
  The CoFlatMapFunction used to jointly transform the two input
  DataStreams
  @return The transformed {@link DataStream}