(define (print-formula f)
  (cond [(var?  f) (display f)]
        [(neg?  f) (display "~") (print-formula (neg-subf f))]
        [(conj? f) (display "(") (print-formula (conj-left f)) (display " & ")
                   (print-formula (conj-rght f)) (display ")")]
        [(disj? f) (display "(") (print-formula (disj-left f)) (display " | ")
                   (print-formula (disj-rght f)) (display ")")]))