Nested set of zones

Nested set of zones --- иерархия вложенных зон.

A set of zones [math]\displaystyle{ A }[/math] of a cf-graph [math]\displaystyle{ G }[/math] forms a nested set of zones of [math]\displaystyle{ G }[/math] if the following two properties hold: [math]\displaystyle{ S_1\cap S_2=\varnothing }[/math] or [math]\displaystyle{ S_1\cap S_2 }[/math] for any [math]\displaystyle{ S_1,S_2\in A }[/math]; and for any zone [math]\displaystyle{ S_1 }[/math] of [math]\displaystyle{ G }[/math] there is such a zone [math]\displaystyle{ S_2\in A }[/math] that [math]\displaystyle{ S_1\subseteq S_2 }[/math] and the zones [math]\displaystyle{ S_1 }[/math] and [math]\displaystyle{ S_2 }[/math] have a common initial node.