class Regexp::Expression::Subexpression

def each_expression(include_self = false, &block)

their parents are also passed to it.
If the block takes two arguments, the indices of the children within
given block.
Traverses the expression, passing each recursive child to the 
def each_expression(include_self = false, &block)
  return enum_for(__method__, include_self) unless block
  if block.arity == 1
    block.call(self) if include_self
    each_expression_without_index(&block)
  else
    block.call(self, 0) if include_self
    each_expression_with_index(&block)
  end
end