... :~$ swipl
Welcome to SWI-Prolog (threaded, 64 bits, version 7.6.0-rc1)
SWI-Prolog comes with ABSOLUTELY NO WARRANTY. This is free software.
Please run ?- license. for legal details.

For online help and background, visit http://www.swi-prolog.org
For built-in help, use ?- help(Topic). or ?- apropos(Word).

?- consult('*/gv1.pro').
true.

?- declare(a,10).
true.

?- declare(b,5).
true.

?- valueof(a,Value).
Value = 10.

?- valueof(b,Value).
Value = 5.

?- bindings.
a -> 10
b -> 5
true.

?- undeclare(b,_).
true.

?- bindings.
a -> 10
true.

?- inc(a).
true.

?- bindings.
a -> 11
true.

?- dec(a).
true.

?- add(a,5).
true.

?- bindings.
a -> 15
true.

?- sub(a,10).
true.

?- bindings.
a -> 5
true.

?- mul(a,5).
true.

?- bindings.
a -> 25
true.

?- div(a,5).
true.

?- bindings.
a -> 5
true.

?- halt.