code: func (list WorkItemLinkList) GetParentIDOf(childID uuid.UUID, linkTypeID uuid.UUID) uuid.UUID {
  	for _, l := range list {
  		if l.TargetID == childID && l.LinkTypeID == linkTypeID {
  			return l.SourceID
  		}
  	}
  	return uuid.Nil
  }
docs: // GetParentIDOf returns the ID (if any) of the parent for the child work item
  // with respect to the link type in the given link list.