Задача на Clips

[2008_student_2013]

Здравствуйте. Помогите пожалуйста исправить ошибки в программе на Clips (программа для построения башни из блоков).
Вот сам код:
(deftemplate block
(field color (type SYMBOL))
(field size (type INTEGER))
(field place (type SYMBOL)) )
(deftemplate on
(field upper (type SYMBOL))
(field lower (type SYMBOL))
(field place (type SYMBOL) (default heap)) )
(deftemplate goal
(field task (type SYMBOL)) )
(deffacts the-facts
(block (color red) (size 10))
(block(color yellow) (size 20))
(block (color blue) (size 30))
)
(defrule begin
(initial-fact) =>
(assert (goal (task find))) )
(defrule pick-up
?my-goal <- (goal (task find))
?my-block <- (block (size ?S1) (place heap))
(not (block (color ?C2) (size ?S2&:(>-?S2 ?S1))
(place heap)))
=>
(modify ?my-block (place hand))
(modify ?my-goal (task build)) )
(defrule place-first
?my-goal <- (goal (task build))
?my-block <- (block (place hand))
(not (block (place tower)))
=>
(modify ?my-block (place tower))
(modify ?my-goal (task find)) )
(defrule put-down
?my-goal <- (goal (task build))
?my-block <- (block (color ?C0)
(place hand))
(block (color ?C1) (place tower))
(not (on (upper ?C2) (lower ?C1)
(place tower)))
=>
(modify ?my-block (place tower))
(assert (on (upper ?C0) (lower ?C1)
(place tower)))
(modify ?my-goal (task find)) )
(defrule stop
?my-goal <- (goal (task find))
(not (block (place heap)))
=>
(retract ?my-goal) )


Выдается ошибка:
[EXPRNPSR3] Missing function declaration for >-?S2.

ERROR:
(defrule pick-up
?my-goal <- (goal (task find))
?my-block <- (block (size ?S1) (place heap))
(not (block (color ?C2) (size ?S2&:(>-?S2
***
FALSE


Что обозначает ошибка и как ее исправить? Подскажите пожалуйста

[2008_student_2013]

Спасибо, помощь уже не нужна) Можно закрыть тему