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

theorem
  { x1,x2,x3,x4 } = { x3,x4,x1,x2 }
proof
  thus { x1,x2,x3,x4 } = { x3,x2,x1,x4 } by Lm8
    .= { x3,x4,x1,x2 } by Th64;
end;
