let l = "l.l"
in let ln = "ln.l"
in idx(l) + 
   (| ln -> l 
      [Exp -> Exp]
      Exp.zero  =  '\z.z'       ;
      Exp.succ  =  '\x.$1'      ;
      Exp.pred  =  '($1 \z.z)'  ;
   |)