reserve x,x1,x2,x3,x4,x5,x6,x7,x8,x9,x10,y for object, X,Z for set;

theorem
  { x1,x2,x3,x4 } = { x2,x4,x3,x1 }
proof
  thus { x1,x2,x3,x4 } = { x2,x3,x1,x4 } by Lm7
    .= { x2,x4,x3,x1 } by Th63;
end;
