Double-dispatch entry point.
A node receives a Visitor instance and then calls a method on a Visitor
that corresponds to a actual type of this Node. This binding however happens
at the compile time and therefore it allows to save on rather expensive
reflection based instanceOf check when decision based on the actual
type of Node is needed, which happens redundantly during traversing AST.
Additional advantage of this pattern is to decouple tree traversal mechanism
from the code that needs to be executed when traversal "visits" a particular Node
in the tree. This leads to a better code re-usability and maintainability.