CompoundNode
A Node that combines multiple other nodes into a single node.
A compound node may have 0.n children. Children are added by calling the accept method.
Accepted children must follow a few invariants. See Node for more information.
There are no general-purpose way of accessing the children after they have been accepted. Instead, this node should be considered as representing the children itself, as a single unit. Subtypes may decide to provide such a feature, however.