let l = "l.l"
in let lb = "lb.l"
in idx(l) + 
   (| lb -> l
      [Exp -> Exp] {
      Exp.true   =  '\x.\y.x'       ;
      Exp.false  =  '\x.\y.y'       ;
      Exp.if     =  '(($1 $2) $3)'  ;
   |)