Returns a node-list containing a single node that is the root of a grove built by parsing an SGML document or subdocument using the SGML property set.  string is the system identifier of the SGML document entity or SGML subdocument entity.  active: is a list of strings specifying the names of the active DTD or LPDs.  At most one DTD shall be active.  If parent: is specified, then the entity to be parsed is an SGML subdocument entity, and the value shall be a singleton node-list in the grove in which the subdocument should be treated as being declared.  This uses the default grove plan, which is determined in an application-dependent manner.