Prolog Beginner: How to unify with arithmetic comparison operators or how to get a set var to range of values

Question

I am new to Prolog. I need to write an integer adder that will add numbers between 0-9 to other numbers 0-9 and produce a solution 0-18. This is what I want to do:

% pseudo code
add(in1, in2, out) :-
    in1 < 10,
    in2 < 10,
    out < 18.

I would like to be able to call it like this:

To check if it is a valid addition:

?- add(1,2,3).
true.
?- add(1,2,4).
false.

With one missing variable:

?- add(X,2,3).
X = 1.
?- add(1,4,X).
X = 5.

With multiple missing variables:

?- add(X,Y,Z).
% Some output that would make sense.  Some examples could be:
X=1, Y=1, Z=2 ;
X=2, Y=1, Z=3 ......

I realize that this is probably a pretty simplistic question and it is probably very straightforward. However, according to the Prolog tutorial I am using:

"Unlike unification Arithmetic Comparison Operators operators cannot be used to give values to a variable. The can only be evaluated when every term on each side have been instantiated."

Answer 1

Solution:

lessThanTen(9).
lessThanTen(8).
lessThanTen(7).
lessThanTen(6).
lessThanTen(5).
lessThanTen(4).
lessThanTen(3).
lessThanTen(2).
lessThanTen(1).
lessThanTen(0).

addSimple(Add1,Add2,Sol) :-
    lessThanTen(Add1),
    lessThanTen(Add2),
    Sol is Add1+Add2.