let l = "l.l"
in let ln = "ln.l"
in letx id2l = idx(l) + 
   (| { Exp.id : "id" ; } -> l
      [Exp -> Exp] {
      Exp.id  =  '\x.x'  ;
   |)
in letx ln2id = idx(l) + 
   (| ln -> src(id2l)
      [Exp -> Exp]
      Exp.zero  =  'id'       ;
      Exp.succ  =  '\x.$1'    ;
      Exp.pred  =  '($1 id)'  ;
   |)
in (id2l o ln2id)