Visual prolog -can I declare a global variable?

Hello!
I want ro use Visual Prolog for my graduation work. Earlier I was
studying Prolog so I know the essentials of the Prolog language. I am
starting studying Visual Prolog, and I need some help.
I cannot understand how to define global variable to pass it between
various forms. Is it possible?
Anyway I need to pass something from one form to another. Or I must
write it to file to store and read from it when the new form is opened?
Vlad

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