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