Grob names must only be unique amongst grobs which
  share the same parent in a gTree.
This function can be used to generate a specification for
  a grob that includes the grob's parent's name
  (and the name of its parent and so on).
For interactive use, it is possible to directly specify
  a path, but it is strongly recommended that this function
  is used otherwise in case the path separator is changed
  in future versions of grid.