[Prolog] How do I declare a variable in Prolog?

In Prolog, variables are declared implicitly when they are used. You don't need to explicitly declare them like in other programming languages. Variables in Prolog are denoted by a string of characters starting with an uppercase letter or an underscore.

For example, if you want to declare a variable X, you can use it directly in a query or a rule like this:

1?- X = 42.
2X = 42.
4?- X = 5 * 8.
5X = 5*8.
7?- Y = 3, X = Y + 2.
8Y = 3,
9X = 3+2.

Here, X and Y are variables that can be assigned any value or expression. The values are determined when the Prolog program is executed or when queries are executed.

Note that variables can be used to unify with other terms, store values, or represent unknown values that need to be found through backtracking and search.