How to uninstantiate a prolog variable ?

[Devt]

A lot of predicates "get" a value and bound it to a
Variable given as argument. But if the Variable is already
instantied, then the predicates "set" the current value of
the Variable.
I don't want a new Variable each time i must get a value,
so how to uninstantiate a Variable or how to deal with this
problem ?

 

[ting138]

I have a list
[a,b,[c,d,[s,d],[c,d]]]
how can I simplify it down to
[a,b,c,d,s,d,c,d]?