theorem Th36:
  n1 >= len p1 & n2 >= len p1 & n3 >= len p1 & n4 >= len p1
  implies crossover(p1,p2,n1,n2,n3,n4) = p1
proof
  assume that
A1: n1 >= len p1 & n2 >= len p1 & n3 >= len p1 and
A2: n4 >= len p1;
  crossover(p1,p2,n1,n2,n3,n4) = crossover(p1,p2,n4) by A1,Th35;
  hence thesis by A2,Th5;
end;
