$use StdIO;

$func  MkArg  eString = ePrefixes;
$func? Select (eXs) eSets = eXs-or-sFail;

Main =
  <MkArg '1234567890ab'> :: e.Prefixes,
  <PrintLN e.Prefixes>,
  <Select () e.Prefixes> :: eXs-or-sFail,
  <PrintLN eXs-or-sFail>;

MkArg { 
  e1 sx = (e1 sx) <MkArg e1>;
  = ;
};

Select \{
  (en) e.Sets, e.Sets : \{
    = en;
    (e1 sa e2) e.Sets1, # \{ en : e3 sa e4; }, <Select (en sa) e.Sets1>;
  };
};