;; ** collect-prop-variables **
;; This function collects the variables of a logical formula from a given list
(define collect-prop-variables
  (lambda (l)
    (cond
      ;; If the list is null, we've gone through everything so return an empty list
      ((null? l) `())
      ;; If the first item in the list is an atom...
      ((atom? (car l))
       (cond
     ;; If it's not a logical operator, keep it and keep going through the rest of the list
     ((eq? (operator? (car l)) #f) (flatten (cons (car l) (collect-prop-variables (cdr l)))))
     ;; Otherwise, ignore it and keep going through the list
     (else (collect-prop-variables (cdr l)))
       )
      )
      ;; If the first item is a list recurse through it and through the rest of the list
      (else (remove-duplicates (flatten (cons (collect-prop-variables (car l)) (collect-prop-variables (cdr l)))))))))