free_term (Cast t s r) = Cast t' s' r
     where t' = free_term t
           s' = mkFreeName s
free_term (Cast t s r) = Cast t' s' r
     where t' = free_term t
           s' = mkFreeName s
free_term (Cast t s r) = Cast t' s' r
     where t' = free_term t
           s' = mkFreeName s