(i) If C is the class of all finite sums of intervals in R (unions of pair-wise disjoint intervals) of the form: (a, β], a, β ∈ R, a < β; (-(, a], a ∈ R; (β, (), β ∈ R, (, then C is a field and σ(C) = B.
(ii) The same is true if C is the class of all finite sums of all kinds of intervals in R.