(node (name root) (type decision) 
      (question "Is the animal warm-blooded?") 
      (yes-node node1) (no-node node2) (answer nil))
(node (name node1) (type decision) 
      (question "Does the animal purr?") 
      (yes-node node3) (no-node node4) (answer nil))
(node (name node2) (type answer) (question nil) 
      (yes-node nil) (no-node nil) (answer snake))
(node (name node3) (type answer) (question nil) 
      (yes-node nil) (no-node nil) (answer cat))
(node (name node4) (type answer) (question nil) 
      (yes-node nil) (no-node nil) (answer dog))