“…For the reader's convenience we remind the definitions of the tube structure and its graph given in [13]. 3, and e 1 , e 2 , ..., e M be M closed segments each connecting two of these points (i.e., each e j = O ij O kj , where i j , k j ∈ {1, ..., N }, i j ̸ = k j ). All points O i are supposed to be the ends of some segments e j .…”