... :~$ 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('*/gv2.pro').
true.

?- declare(a,10).
true.

?- declare(b,10).
true.

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

?- undeclare(b,_).
true.

?- bindings.
a -> 10
true.

?- declare(b,5).
true.

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

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

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

?- add(a,b,sum).
true.

?- bindings.
a -> 10
b -> 5
sum -> 15
true.

?- sub(a,b,sub).
true.

?- mul(a,b,prod).
true.

?- div(a,b,quote).
true.

?- pow(a,b,power).
true.

?- bindings.
a -> 10
b -> 5
sum -> 15
sub -> 5
prod -> 50
quote -> 2
power -> 100000
true.

?- halt.