class Sass::Tree::Node

def <<(child)
  if msg = invalid_child?(child)
    raise Sass::SyntaxError.new(msg, child.line)
  end
  @children << child
end

def ==(other)
  self.class == other.class && other.children == children
end

def _perform(environment)
  node = dup
  node.perform!(environment)
  node
end

def balance(*args)
  res = Haml::Shared.balance(*args)
  return res if res
  raise Sass::SyntaxError.new("Unbalanced brackets.", line)
end

def filename
  @filename || @options[:filename]
end

def initialize
  @children = []
end

def interpolate(text, environment)
  res = ''
  rest = Haml::Shared.handle_interpolation text do |scan|
    escapes = scan[2].size
    res << scan.matched[0...-2 - escapes]
    if escapes % 2 == 1
      res << "\\" * (escapes - 1) << '#{'
    else
      res << "\\" * [0, escapes - 1].max
      res << Script::Parser.new(scan, line, scan.pos - scan.matched_size, filename).
        parse_interpolated.perform(environment).to_s
    end
  end
  res + rest
end

def invalid_child?(child)
  false
end

def invisible?; false; end

def last
  children.last
end

def options=(options)
  children.each {|c| c.options = options}
  @options = options
end

def perform(environment)
  environment.options = @options if self.class == Tree::Node
  _perform(environment)
rescue Sass::SyntaxError => e; e.add_metadata(filename, line)
end

def perform!(environment)
  self.children = perform_children(Environment.new(environment))
end

def perform_children(environment)
  children.map {|c| c.perform(environment)}.flatten
end

def render
  perform(Environment.new).to_s
end

def style
  @options[:style]
end

def to_s
  result = String.new
  children.each do |child|
    if child.is_a? PropNode
      raise Sass::SyntaxError.new('Properties aren\'t allowed at the root of a document.', child.line)
    else
      next if child.invisible?
      child_str = child.to_s(1)
      result << child_str + (style == :compressed ? '' : "\n")
    end
  end
  result.rstrip!
  return "" if result.empty?
  return result + "\n"
rescue Sass::SyntaxError => e; e.add_metadata(filename, line)
end

def to_sass(tabs = 0, opts = {})
  result = ''
  children.each do |child|
    result << "#{'  ' * tabs}#{child.to_sass(0, opts)}\n"
  end
  result
end