fn padding_constraint(&self) -> Constraint<F> {
        let zero = FieldElement::zero();
        Constraint {
            constraint_type: ConstraintType {
                ql: zero.clone(),
                qr: zero.clone(),
                qm: zero.clone(),
                qo: zero.clone(),
                qc: zero,
            },
            hint: None,
            l: self.null_variable(),
            r: self.null_variable(),
            o: self.null_variable(),
        }
    }
A dummy constraint meant to be used as padding.
crates/provers/plonk/src/constraint_system/mod.rs