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

theorem Th59:
  { x1,x2,x3 } = { x2,x3,x1 }
proof
  thus { x1,x2,x3 } = { x2,x3 } \/ { x1 } by Th2
    .= { x2,x3,x1 } by Th3;
end;
