WalkFunc is like Walk but accepts just a callback function.

GoToNext is the default traversal of every node.

SkipChildren tells walker to skip all children of current node.

Terminate tells walker to terminate the traversal.

NodeVisitor is a callback to be called when traversing the syntax tree.

NodeVisitorFunc casts a function to match NodeVisitor interface.

WalkStatus allows NodeVisitor to have some control over the tree traversal.