A visitor is provided to visit, it contains the collection of relevant functions to be called during the visitor's traversal.

Given a visitor instance, if it is leaving or not, and a node kind, return the function the visitor runtime should call.

visit() will walk through an AST using a depth first traversal, calling the visitor's enter function at each node in the traversal, and calling the leave function after visiting that node and all of its child nodes.

Creates a new visitor instance which delegates to many visitors to run in parallel. Each visitor will be visited for each node before moving on.