  Given a compact set K in the plane, which contains no triple of points
forming a vertical and a horizontal segment, and a continuous real-valued map f
on K, we give a construction of real-valued continuous maps of one variable g,h
such that f(x,y)=g(x)+h(y) for all points (x,y) from K. This provides a
constructive proof of a part of Sternfeld's theorem on basic embeddings in the
plane. In the proof we construct a sequence of finite graphs, which provide an
arbitrarily good approximation of the set K.
