;; IS-SUBSET
;; helper function
(defun is-subset (small big) 
  (setf small (remove-if #'(lambda (symbol) (find symbol big :test 'eql)) small))
  (eql small nil))

;; FORWARD-CHAIN-REC
;; recursive call function
(defun forward-chain-rec (literals rules)
  (let ((inferred literals))
	(dolist (rule rules)
	  (if (is-subset (car rule) literals) 
		  (setf inferred (append inferred (list (cadr rule))))))
	(if (equalp (remove-duplicates inferred) literals) 
		(remove-duplicates inferred)
		(forward-chain-rec (remove-duplicates inferred) rules))))

;; FORWARD-CHAIN
;; Main function. Usage should be self-explanatory.
(defun forward-chain (kb) 
  (let* ((literals (remove-if-not 'atom kb))
		 (rules (remove-if 'atom kb))
		 (inferred (forward-chain-rec literals rules)))
	(remove-if #'(lambda (symbol) (find symbol literals :test 'eql)) inferred)))