free_formula (Membership t s r) = Membership t' s' r
     where t' = free_term t
           s' = mkFreeName s
free_formula (Membership t s r) = Membership t' s' r
     where t' = free_term t
           s' = mkFreeName s
free_formula (Membership t s r) = Membership t' s' r
     where t' = free_term t
           s' = mkFreeName s