tree = ^ node;

SyntaxClass = ( constant, intcon, variable, func,
                predicate, negate, rule, progrm,
                list );
                
node = record case tag :SyntaxClass of
          constant    :( cid :alfa );                 { eg. fred }
          intcon      :( n:integer );                 { eg. 99 }
          variable    :( vid :alfa; index :integer ); { eg. X }
          func, predicate
                      :( id :alfa; params:tree );     { eg. h(1,k(p,X)) }
          negate      :( l :tree );                   { eg. not p(7) }
          rule        :( lhs, rhs :tree );            { eg. p <= q and r. }
          progrm      :( facts, query :tree );
          list        :( hd, tl :tree )
end;

{\fB Syntactic Types. \fP}