code: void addInitialSubtask(final ExecutionVertex ev) {
  
  		if (ev == null) {
  			throw new IllegalArgumentException("Argument ev must not be null");
  		}
  
  		if (this.initialGroupMemberAdded.compareAndSet(false, true)) {
  			this.groupMembers.add(ev);
  		}
  	}
docs: Adds the initial execution vertex to this group vertex.
  
  @param ev
  The new execution vertex to be added.