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