Trees and graphs are closely related: An ordered tree is a special form of graph with the properties of <hi xmlns="http://www.tei-c.org/ns/1.0" rend="italic">a)</hi> it is a directed graph without cycles, <hi xmlns="http://www.tei-c.org/ns/1.0" rend="italic">b)</hi> has one designated root node and <hi xmlns="http://www.tei-c.org/ns/1.0" rend="italic">c)</hi> any node has exactly one parent node.