A List is a collection of elements where:
A Tree has property (1), but (2) is relaxed:
(2') each element has some number of successors
N-ary Tree: each element has at most N successors.
Binary Tree: N = 2. General trees can be represented using binary trees.
Root of a tree: the unique node with no predecessor.
Leaf of a tree: any node with no successor.
Children of a node: its successors.
Parent of a node: its unique predecessor.
Siblings: children of the same parent.