Subsets of Three Properties
lDAG: acyclic, but
lmay have multiple roots,
lmay have fan-in
lForest: acyclic, and no fan-in but
lmay have multiple roots
lA forest can also be characterized as a 
collection of disjoint trees.
Each tree could be identified with its root.
collection of disjoint trees. Each tree could be identified with its root.