 For formulations restricted to Set-Fmla0, and thus suitable for the intuitionistic sequent calculus, we require that Δ in (∧ Left) contains at most one formula, and for (∨ Left) and (→ Left), that Δ ∪ Δ′ contains at most one formula. Indeed, one easily sees that using the structural rules mentioned earlier and the operational rules described here, there will always be exactly one formula, since proofs all begin with (Id) and no rule removes all formulas from the right-hand side (or ‘succedent’) in the transition from premiss sequents to conclusion sequent. This changes if we add the rules for negation, here giving both the left and the right insertion rules, initially for the Set-Set case: