theorem Th26:
  crossover(p1,p2,n1,n2,n3) = crossover(p1,p2,n3,n1,n2)
proof
  crossover(p1,p2,n1,n2,n3) = crossover(p1,p2,n1,n3,n2) by Th25;
  hence thesis by Th25;
end;
