class Regexp::Expression::Subexpression

def strfregexp_tree(format = '%a', include_self = true, separator = "\n")
  output = include_self ? [self.strfregexp(format)] : []
  output += map {|exp, index|
    exp.strfregexp(format, (include_self ? 1 : 0), index)
  }
  output.join(separator)
end