procedure printtree(t:tree; e:Env);
   var v :tree;

   procedure printId(id:alfa);
      var i:integer;
   begin i:=1;
         while i<=10 do
           if id[i]=' ' then i:=11 else begin write(id[i]); i:=i+1 end
   end;

   procedure printTail( t :tree );
   begin if t <> nil then
         begin write(', '); printtree(t^.hd, e); printTail(t^.tl) end
   end;

begin{printtree}
   if t<>nil then
      case t^.tag of
         variable:if bound(t, v, e) then printtree(v, e)
                  else
                  begin printId(t^.vid);
                        if t^.index<>0 then write('_', t^.index:1)
                  end;
         constant:printId(t^.cid);
         intcon:  write(t^.n:1);
         predicate, func:
                  begin printId(t^.id);
                        if t^.params<>nil then
                        begin write('('); printtree(t^.params, e); write(')')
                        end
                  end;
         negate:  begin write(' not '); printtree(t^.l, e) end;
         rule:    begin printtree(t^.lhs, e);
                        if t^.rhs<>nil then
                        begin write(' <= '); printtree(t^.rhs, e)
                        end;
                        writeln('.')
                  end;
         list:    begin printtree(t^.hd, e); printTail(t^.tl) end;
         progrm:  begin printtree(t^.facts, e); writeln;
                        write('?'); printtree(t^.query, e); writeln
                  end
      end{case};
end{printtree};

{\fB Print a Parse Tree. \fP}