How to use not in gprolog?

[Carfield Yim]

Hi, I need to find the negation of a clause, e.g.:

e.g: if Z is a member of list T and Y is not a member of T, member(Z,T) should 
return true and member(Y,T) should return false.
I would like to get the negation of this clause, i.e.: not(member(Z,T))
return false and not(member(Y,T)) return true. But it doesn't work. So I
try member(X,T)\=true. and once(member(X,T))\=true. But both don't work.

So I would like to ask how to get the negation of a clause?

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Carfield Yim, visit my homepage at http://www.carfield.com.hk