simple question (getting error )

[fatemeh]

hi every one,I'm just getting familiar with clips so I'm sorry for my simple question.
I'm trying to write a simple code in clips but I'm getting this error which i can't find what the problem is.
I would be grateful if any one could help.

Defining deffunction: user-Question
Defining defrule: Rule0
[PRNTUTIL2] Syntax Error: Check appropriate syntax for defrule.

ERROR:
(defrule MAIN::Rule0
(not (tank-is-full ?)
=>

this is part of my code:

;;;*
;;; Functions
;;;***
(deffunction user-Question(?question ?answer $?values)
(printout t ?question)
(bind ?answer(read))
(stringp ?answer)
=>
(bind ?answer(lowcase ?answer))

(while (not (member$ ?answer ?values)) do
(printout t ?question)
(bind ?answer(read))
(stringp ?answer)
=>
(bind ?answer(lowcase ?answer))
)
?answer
)

;;;***
;;; ENGINE STATE RULES
;;;***

(defrule Rule0
(not (tank-is-full ?)
=>
(bind ?tank-is-full user-Question("Is there any petrol in the tank(Yes|No)?"yes YES Yes NO no No)
(assert(tank-is-full ?tank-is-full)
)
;;;****

[Gary Riley]

The rule has multiple missing/misplaced parentheses:

(defrule Rule0
   (not (tank-is-full ?))
   =>
   (bind ?tank-is-full 
      (user-Question "Is there any petrol in the tank(Yes|No)?" 
                     yes YES Yes NO no No))
   (assert (tank-is-full ?tank-is-full)))