expr_fvs (App fun arg) fv_cand in_scope acc =
  (expr_fvs fun `unionFV` expr_fvs arg) fv_cand in_scope acc
expr_fvs (App fun arg) fv_cand in_scope acc =
  (expr_fvs fun `unionFV` expr_fvs arg) fv_cand in_scope acc
expr_fvs (App fun arg) fv_cand in_scope acc =
  (expr_fvs fun `unionFV` expr_fvs arg) fv_cand in_scope acc