theorem Th28:
  All('not' Ex(a,A,G),B,G) '<' 'not' All(All(a,B,G),A,G)
proof
  All('not' Ex(a,A,G),B,G) = All(All('not' a,A,G),B,G) by BVFUNC_2:19;
  hence thesis by Th27;
end;
