Задача о мере объединения отрезков на плоскости с ограничением на множество их концов

Some time ago M.A. Patrakeev asked the following question. Let $A$ and $B$ be zero-measure subsets of the unit segment. Let $\varphi$ be bijection between $A$ and $B$. Denote by $S(A,B,\varphi)$ the union of all segments in the plane with the endpoints $(a,0)$ and $(\varphi(a),1)$ for some $a\in A$. The question is what the measure of the set $S(A,B,\varphi)$. We answer this question.