*slime-scratch*

(defun pythagorean (a b)
  "Find the 3rd line in a right angle. a^2 + b^2 = c^2 and a > c > b"
  (declare (real a b))
  (if (> a b) 'f
      (sqrt (+ (expt a 2) (expt b 2)))))

(defun reverse-pythagorean (c b)
  "Reverse of `pythagorean' c^2 - b^2 = a^2 and c < b < a"
  (declare (real b c))
  (if (> b c) 'f 
      (sqrt (- (expt c 2) (expt b 2)))))

(defun pythagoreanp (a b c)
  "Returns t or nil if it's a correct pythagorean or not."
  (declare (real a b c))
  (and (> a b) (> b c) = (pythagorean a b) c) (= (reverse-pythagorean c b) a))