Flat forest

Flat forest --- плоский лес.

A. flat forest of a graph [math]\displaystyle{ G = (V,E) }[/math] is a forest [math]\displaystyle{ F = (V,E') }[/math] such that [math]\displaystyle{ E' \subset E }[/math] and each tree in [math]\displaystyle{ F }[/math] has height at most 1. Any zero-degree node in [math]\displaystyle{ G }[/math] is zero-degree in [math]\displaystyle{ F }[/math], too.