class Regexp::Expression::Subexpression

def traverse(include_self = false, &block)

Returns self.

- For terminal expressions, :visit is called once.

:exit upon exiting it.
- For subexpressions, :enter upon entering the subexpression, and

The event argument is passed as follows:

the expression, and the index of the expression within its parent.
block for each expression with three arguments; the traversal event,
Traverses the subexpression (depth-first, pre-order) and calls the given 
def traverse(include_self = false, &block)
  return enum_for(__method__, include_self) unless block_given?
  block.call(:enter, self, 0) if include_self
  each_with_index do |exp, index|
    if exp.terminal?
      block.call(:visit, exp, index)
    else
      block.call(:enter, exp, index)
      exp.traverse(&block)
      block.call(:exit, exp, index)
    end
  end
  block.call(:exit, self, 0) if include_self
  self
end