docs: def locateChild(self, ctx, segments):
          """
          Look up a shared item for the role viewing this SharingIndex and return a
          L{PublicAthenaLivePage} containing that shared item's fragment to the
          user.
  
          These semantics are UNSTABLE.  This method is adequate for simple uses,
          but it should be expanded in the future to be more consistent with
          other resource lookups.  In particular, it should allow share
          implementors to adapt their shares to L{IResource} directly rather than
          L{INavigableFragment}, to allow for simpler child dispatch.
  
          @param segments: a list of strings, the first of which should be the
          shareID of the desired item.
  
          @param ctx: unused.
  
          @return: a L{PublicAthenaLivePage} wrapping a customized fragment.
          """
          shareID = segments[0].decode('utf-8')
  
          role = self.webViewer.roleIn(self.userStore)
  
          # if there is an empty segment
          if shareID == u'':
              # then we want to return the default share.  if we find one, then
              # let's use that
              defaultShareID = getDefaultShareID(self.userStore)
              try:
                  sharedItem = role.getShare(defaultShareID)
              except sharing.NoSuchShare:
                  return rend.NotFound
          # otherwise the user is trying to access some other share
          else:
              # let's see if it's a real share
              try:
                  sharedItem = role.getShare(shareID)
              # oops it's not
              except sharing.NoSuchShare:
                  return rend.NotFound
  
          return (self.webViewer.wrapModel(sharedItem),
                  segments[1:])