<!ELEMENT kb            (hn*) >
<!ELEMENT hn            (relationship, relationship*) >
<!ELEMENT relationship  (relator,     (ind | var | struc)*) >
<!ELEMENT struc         (constructor, (ind | var | struc)*) >
<!ELEMENT relator       (#PCDATA) >
<!ELEMENT constructor   (#PCDATA) >
<!ELEMENT ind           (#PCDATA) >
<!ELEMENT var           (#PCDATA) >