Returns a node-list containing a single node that is the root of a
grove built by parsing the prolog of an SGML document. string is
the system identifier of the SGML document entity. This is built
using the default grove plan modified to exclude the instabs module.
|This procedure is typically used to specify the subgrove:
argument to the subgrove-spec: procedure when the
source and result groves have different DTDs.|