theorem Th53:
  crossover(p1,p2,n1,n2,n3,n4,n5,n6) is Individual of S
proof
  reconsider q1=crossover(p1,p2,n1,n2,n3,n4,n5), q2=crossover(p2,p1,n1,n2,n3,
  n4,n5) as Individual of S;
  crossover(q1,q2,n6) is Individual of S;
  hence thesis;
end;
