! Missing } inserted.
<inserted text>
                }
l.47     Test $Id$
                   , $Id{A}$ and $Id{A,B}$ inline.

documentclass{article}
usepackage{amsmath}
usepackage{xparse} % loads expl3
%See interface3.pdf

ExplSyntaxOn

NewDocumentCommand Id {O{}}
  {
     Id:n #1
  }

cs_new:Npn Id:n #1
  {
    clist_set:Nn l_tmpa_clist {#1}
    int_case:nn
      {clist_count:N l_tmpa_clist}
      {
        {0}
        {
          mathop{Id}
        }
        {1}
        {
          mathop{Id}_{#1}
        }
        {2}
        {
          mathop{Id}
            ^{clist_item:Nn l_tmpa_clist 1}
            _{clist_item:Nn l_tmpa_clist 2}
        }
      }
      {
          msg_error:nn {shmuel} {More than one argument for {backslash}Id}
      }
  }

begin{document}

ExplSyntaxOn

Test $Id$, $Id{A}$ and $Id{A,B}$ inline.
begin{equation}
  Test Id, \
   Id{A}, \
   Id{A,B}
end{equation}

end{document}