theorem Th6:
  crossover(p1,p2,n1,n2) is Individual of S
proof
  reconsider q1=crossover(p1,p2,n1),q2 = crossover(p2,p1,n1) as Individual of
  S;
  crossover(q1,q2,n2) is Individual of S;
  hence thesis;
end;
