\documentclass{article}


\ExplSyntaxOn


\clist_new:N \copied_clist
\seq_new:N \saved_seq

%remove left  -> push on stack
% call recursive
% add left to 'extra'
% call recursive
% remove left from 'extra'
% add from stack to left


\cs_generate_variant:Nn \clist_remove_all:Nn { N V }

\cs_new:Npn \clist_use_powerset_aux:Nn #1 #2 
{
  \clist_if_empty:NTF \copied_clist
    {
      #2
    }
    {

      \clist_get:NN \copied_clist \l_tmpa_tl % gets first element of copied list into local
      \seq_push:NV \saved_seq \l_tmpa_tl     % pushes local value onto stack
      \clist_pop:NN \copied_clist { \l_tmpa_tl } % removes from copied list

      \clist_use_powerset_aux:Nn #1 {#2}
      
      \seq_get:NN \saved_seq \l_tmpa_tl
      \clist_put_left:NV #1 \l_tmpa_tl

      \clist_use_powerset_aux:Nn #1 {#2}

      \seq_get:NN \saved_seq \l_tmpa_tl
      \clist_remove_all:NV #1 \l_tmpa_tl
      \clist_push:NV \copied_clist \l_tmpa_tl
      \seq_pop:NN \saved_seq \l_tmpa_tl
    }
}

\cs_new:Npn \clist_use_powerset:Nn #1 #2
{
  \clist_set_eq:NN \copied_clist #1
  \clist_clear:N #1
  \clist_remove_duplicates:N \copied_clist

  \clist_use_powerset_aux:Nn #1 {#2}

  \clist_set_eq:NN #1 \copied_clist
}

\clist_new:N  \test_clist
\tl_set:Nn \test_clist {1,2,3,4,5,6,7,8,9}

\def\foo {
  \clist_use_powerset:Nn \test_clist
    {
%      reached~end:~
%      \clist_use:Nn \test_clist  {,} \newline
      \stepcounter{testval}
    }
  \test_clist
}

\ExplSyntaxOff


\begin{document}

\newcounter{testval}


\noindent \foo

\thetestval

\end{document}
