Function less_than 

pub fn less_than<Var>(
    terms: impl Into<Box<[Var]>>,
    rhs: i32,
    constraint_tag: ConstraintTag,
) -> impl NegatableConstraint
where
    Var: IntegerVariable + 'static,

Create the NegatableConstraint ∑ terms_i < rhs.

Its negation is ∑ terms_i <= rhs