free_term (Mixfix_sorted_term s r) = Mixfix_sorted_term s' r
     where s' = mkFreeName s
free_term (Mixfix_sorted_term s r) = Mixfix_sorted_term s' r
     where s' = mkFreeName s
free_term (Mixfix_sorted_term s r) = Mixfix_sorted_term s' r
     where s' = mkFreeName s