expr_fvs (Let (Rec pairs) body) fv_cand in_scope acc
  = addBndrs (map fst pairs)
             (mapUnionFV rhs_fvs pairs `unionFV` expr_fvs body)
               fv_cand in_scope acc
expr_fvs (Let (Rec pairs) body) fv_cand in_scope acc
  = addBndrs (map fst pairs)
             (mapUnionFV rhs_fvs pairs `unionFV` expr_fvs body)
               fv_cand in_scope acc
expr_fvs (Let (Rec pairs) body) fv_cand in_scope acc
  = addBndrs (map fst pairs)
             (mapUnionFV rhs_fvs pairs `unionFV` expr_fvs body)
               fv_cand in_scope acc