free_term (Mixfix_braced ts r) = Mixfix_braced ts' r
     where ts' = map free_term ts
free_term (Mixfix_braced ts r) = Mixfix_braced ts' r
     where ts' = map free_term ts
free_term (Mixfix_braced ts r) = Mixfix_braced ts' r
     where ts' = map free_term ts