Sibling

One end of a Pcs chain: the two children-list boundary markers from Lindholm's paper ("A Three-way Merge for XML Documents", DocEng '04, section 2.3) - ⊣ (nothing precedes the first child) and ⊢ (nothing follows the last) - plus an ordinary node, identified by its label (see Pcs.label). The markers can't be edited, so every real child is guaranteed both a predecessor and a successor, even at either end of the list.