If there are children, get the {@code i}th value indexed from 0.
How many children are there? If there is none, then this node represents a leaf node.
The parent of this node. If the return value is null, then this node is the root of the tree.
This method returns whatever object represents the data at this note. For example, for parse trees, the payload can be a {@link Token} representing a leaf node or a {@link RuleContext} object representing a rule invocation. For abstract syntax trees (ASTs), this is a {@link Token} object.
Print out a whole tree, not just a node, in LISP format {@code (root child1 .. childN)}. Print just a node if this is a leaf.
The basic notion of a tree has a parent, a payload, and a list of children. It is the most abstract interface for all the trees used by ANTLR.