code: public <K> PartitionOperator<T> partitionCustom(Partitioner<K> partitioner, int field) {
  		return new PartitionOperator<>(this, new Keys.ExpressionKeys<>(new int[] {field}, getType()), clean(partitioner), Utils.getCallLocationName());
  	}
docs: Partitions a tuple DataSet on the specified key fields using a custom partitioner.
  This method takes the key position to partition on, and a partitioner that accepts the key type.
  
  <p>Note: This method works only on single field keys.
  
  @param partitioner The partitioner to assign partitions to keys.
  @param field The field index on which the DataSet is to partitioned.
  @return The partitioned DataSet.