Subnode properties

• Nodal property declared in property set to be one of three kinds: subnode, refnode, extnode
• subnode properties connect all the nodes in a grove into a tree-like structure:
  • value of subnode property is a node or list of nodes in the same grove
  • there is a unique node in a grove that does not occur in the value of a subnode property of any node -- the grove root
  • for every node, X, other than the grove root, there is a unique node Y such that Y has a subnode property P whose value includes X; Y is called the origin of X; P is the occurrence mode of X
• This is needed to make the DSSSL transformation language work