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

theorem
  { x1,x1,x1 } = { x1 }
proof
  thus { x1,x1,x1 } = { x1,x1 } by Th30
    .= { x1 } by Th29;
end;
