docs: public void setCombinable(boolean combinable) {
  		// sanity check
  		if (combinable && !GroupCombineFunction.class.isAssignableFrom(this.userFunction.getUserCodeClass())) {
  			throw new IllegalArgumentException("Cannot set a UDF as combinable if it does not implement the interface " +
  					GroupCombineFunction.class.getName());
  		} else {
  			this.combinable = combinable;
  		}
  	}