Let X be a second
countable topological space, A a σ-field of subsets of X containing all open
sets and μ a finile positive measure on A, such that (X,A,μ) is a complete
measure space and μ(U) > 0 for every nonempty open U ⊂ X. Then there
exists a lifting ϕ : A → A which satisfies U ⊂ ϕ(U) for every open subset
U ⊂ X.