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'''Point-tree hypergraph''' --- дерево-точечный гиперграф. 

A hypergraph <math>H</math> is called a ''' point-tree hypergraph''' if it is
obtained from a '' bipartite'' graph by replacing, in each edge, the
point in one side of the graph (the same side for all edges) by a
tree. More formally, <math>H</math> is ''' point-tree''', if for some set <math>X</math> and a
tree, whose vertex set is disjoint with <math>X</math>, each edge <math>e \in H</math> is of
the form <math>\{x\} \cup V(t)</math>, where <math>x = x(e) \in X</math>and <math>t = t(e)</math> is a
subtree of <math>T</math>. For such a hypergraph we denote by <math>\sigma(H)</math> the
number <math>w(H,F)</math>, where <math>F = F(H) = \{\{x(e)\}: \; e \in H\} \cup
\{V(t(e)): \; e \in H\}</math>.