(if (eq? 'hazel 'blue)
  '()
  (let ((mom-path
          (path-to-blue-eyes
            (ft-mom
              (make-ft
                'robby
                'hazel
                (make-ft
                  'alice
                  'green
                  (make-ft 'emily 'brown 'unknown 'unknown)
                  (make-ft 'bruce 'blue 'unknown 'unknown))
                (make-ft 'bill 'brown 'unknown 'unknown)))))
        (dad-path
          (path-to-blue-eyes
            (ft-dad
              (make-ft
                'robby
                'hazel
                (make-ft
                  'alice
                  'green
                  (make-ft 'emily 'brown 'unknown 'unknown)
                  (make-ft 'bruce 'blue 'unknown 'unknown))
                (make-ft 'bill 'brown 'unknown 'unknown))))))
    (cond
     ((and mom-path dad-path) (cons 'mom mom-path))
     ((and mom-path (not dad-path)) (cons 'mom mom-path))
     ((and dad-path (not mom-path)) (cons 'dad dad-path))
     (else #f))))