?- consult('gv.pro').
true.

?- declare(x, 9).
true.

?- declare(y, 5).
true.

?- value_of(x, Value).
Value = 9.

?- bind(x, 3).
true.

?- display_bindings.
y: 5
x: 3
true.

?- inc(x).
true.

?- inc(y).
true.

?- display_bindings.
x: 4
y: 6
true.

?- dec(x).
true.

?- dec(y).
true.

?- display_bindings.
x: 3
y: 5
true.

?- add(x, 5).
true.

?- display_bindings.
y: 5
x: 8
true.

?- sub(y, 7).
true.

?- display_bindings.
x: 8
y: -2
true.

?- add(y, 12).
true.

?- display_bindings.
x: 8
y: 10
true.

?- sub(y, 1).
true.

?- display_bindings.
x: 8
y: 9
true.

?- declare(z, 2).
true.

?- display_bindings.
x: 8
y: 9
z: 2
true.

?- undeclare(z).
true.

?- display_bindings.
x: 8
y: 9
true.

?- halt.