A set implementation in LaTeX?

Question

Consider the following straightforward implementation of a list in latex:

\newcommand{\add@to@list}[2]{%
  \ifx#2\@empty%
    \xdef#2{#1}%
  \else%
    \xdef#2{#2,#1}%
  \fi%
}%

I wonder if there is a simple way to implement a set (list with no repeated elements) ?

Answer 1

This seems to work:

\newcommand{\add@to@set}[2]{%
   \ifx#2\@empty%
      \xdef#2{#1}%
   \else%
      \@expandtwoargs\@removeelement{#1}{#2}{#2}%
      \xdef#2{#2,#1}%
   \fi%
}%