; heron.l

(defun semiperimeter (a b c) 
  (/ (+ a b c) 2)
)

(defun heron (a b c)
  (sqrt  
    (* 
      (semiperimeter a b c) 
      (- (semiperimeter a b c) a) 
      (- (semiperimeter a b c) b)
      (- (semiperimeter a b c) c)
    )
  )
)