code: @Nullable
  	MultiTaskSlot getUnresolvedRootSlot(AbstractID groupId) {
  		for (MultiTaskSlot multiTaskSlot : unresolvedRootSlots.values()) {
  			if (!multiTaskSlot.contains(groupId)) {
  				return multiTaskSlot;
  			}
  		}
  
  		return null;
  	}
docs: Gets an unresolved slot which does not yet contain the given groupId. An unresolved
  slot is a slot whose underlying allocated slot has not been allocated yet.
  
  @param groupId which the returned slot must not contain
  @return the unresolved slot or null if there was no root slot with free capacities