Equivalence of sparse and Carleson coefficients for general sets
  We remark that sparse and Carleson coefficients are equivalent for every
countable collection of Borel sets and hence, in particular, for dyadic
rectangles, the case relevant to the theory of bi-parameter singular integrals.
The key observation is that a dual refomulation by I. E. Verbitsky for
Carleson coefficients over dyadic cubes holds also for Carleson coefficients
over general sets. We give a simple proof for this reformulation.
