free_formula (Relation f1 c f2 r) = Relation f1' c f2' r
     where f1' = free_formula f1
           f2' = free_formula f2
free_formula (Relation f1 c f2 r) = Relation f1' c f2' r
     where f1' = free_formula f1
           f2' = free_formula f2
free_formula (Relation f1 c f2 r) = Relation f1' c f2' r
     where f1' = free_formula f1
           f2' = free_formula f2