Defines the classes and properties to be used in the construction of
groves from the parsing of SGML documents.

Classes and properties are classified as follows:
  o Abstract or SGML document string (SDS)
  o SGML declaration, document prolog, or document instance
  o Required only for support of datatag, rank, shortref, link, subdoc,
    fpi, architectures, notset, fsi, genarc, pelement.

ESIS corresponds roughly to the combination of baseabs (base abstract),
prlgabs0, and instabs (instance abstract).