 A polytree is a directed graph formed by orienting the edges of a free tree . Every polytree is a DAG . In particular , this is true of the arborescences formed by directing all edges outwards from the roots of a tree . 
 A polytree <a href='drop_aux' title='1'><mark>is</mark></a>a directed graph formed by orienting the edges of a free tree . Every polytree <a href='drop_aux' title='2'><mark>is</mark></a>a DAG . In particular , this <a href='drop_aux' title='3'><mark>is</mark></a>true of the arborescences formed by directing all edges outwards from the roots of a tree . 
A polytree a directed graph formed by orienting the edges of a free tree . Every polytree a DAG . In particular , this tru of the arborescences formed by directing all edges outwards from the roots of a tree .