in set theory, the combinatorial principle that, for every stationary 𝑆⊂ω₁, there exists a sequence of sets 𝐴_𝛿 (𝛿∈𝑆) such that 𝐴_𝛿 is a cofinal subset of 𝛿 and every unbounded subset of ω₁ is contained in some 𝐴_𝛿