“…In general, if f : M → R is an arbitrary smooth function with isolated critical points, then a certain part of its "combinatorial symmetries" is reflected by the so-called Kronrod-Reeb graph ∆ f , see, e.g., [2,3,18,19,26,33,35,38] and Section 2.3. Such a graph is obtained by shrinking each connected component of each level set f −1 (c), c ∈ R, of f into a point.…”