JESS: access to specific slots

hi guys,

can anyone tell me how to access a specific slot in the jesstab in protege
3.4.6 ?

i.e.
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(defrule find-something "finds something"
?p <- (object (is-a class-name)
(slot-name ?s)
(slot-name ?v))
=>
(printout t "bla bla " (instance-name ?p) " bla bla " (instance-name ?v) "
 bla. "crlf)
)
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the rule above works fine but when i try to do the following, then it doesnt
work anymore

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(defrule find-something "finds something"
?p <- (object (is-a class-name)
(slot-name specific-instance-of-a-class)
(slot-name ?v))
=>
(printout t "bla bla " (instance-name ?p) " bla bla " (instance-name ?v) "
 bla. "crlf)
)
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thank you for your time