theorem Th43:
  'not' Ex(Ex(a,A,G),B,G) '<' Ex('not' Ex(a,B,G),A,G)
proof
  'not' Ex(Ex(a,A,G),B,G) '<' 'not' All(Ex(a,B,G),A,G) & Ex('not' Ex(a,B,G
  ),A, G) = Ex(All('not' a,B,G),A,G) by Th34,BVFUNC_2:19;
  hence thesis by Th19;
end;
