\{ x \} = \{ x_1, x_2, x_3 ... x_n \} \rightarrow \{ rg(x) \} =

 \{ x_{i_1}, x_{i_2}, x_{i_3} ... x_{i_n} \}, \, {\rm where} \, x_{i_1} \leq x_{i_2} \leq x_{i_3} ...  \leq x_{i_n}