It is sometimes convenient to group nodes in a grove in an application-dependent manner.  This is done by using nodes in the grove as the source for a further parse, called an auxiliary parse.  A grove created by an auxiliary parse is called an auxiliary grove. The grove parsed to create the auxiliary grove is called the source grove of the auxiliary grove.  Each node in an auxiliary grove has an intrinsic urefnode property, source, that points to those nodes in the source grove from which it was derived.