theorem Th28:
  crossover(p1,p2,n1,n2,n3,n4) is Individual of S
proof
  reconsider q1=crossover(p1,p2,n1,n2,n3), q2=crossover(p2,p1,n1,n2,n3) as
  Individual of S;
  crossover(q1,q2,n4) is Individual of S;
  hence thesis;
end;
