pub fn assert_eq(&mut self, v1: &Variable, v2: &Variable) {
        self.add_constraint(Constraint {
            constraint_type: ConstraintType {
                ql: FieldElement::one(),
                qr: -FieldElement::one(),
                qm: FieldElement::zero(),
                qo: FieldElement::zero(),
                qc: FieldElement::zero(),
            },
            l: *v1,
            r: *v2,
            o: self.null_variable(),
            hint: None,
        });
    }
Adds a constraint to enforce that `v1` is equal to `v2`.
crates/provers/plonk/src/constraint_system/conditional.rs