theorem Th7:
  crossover(p1,p2,0,n) = crossover(p2,p1,n)
proof
  crossover(p1,p2,0,n) = crossover(p2,crossover(p2,p1,0),n) by Th4
    .= crossover(p2,p1,n) by Th4;
  hence thesis;
end;
