class Sass::Tree::MixinNode

def perform!(environment)
  handle_include_loop!(environment) if environment.mixins_in_use.include?(@name)
  original_env = environment
  original_env.push_frame(:filename => filename, :line => line)
  original_env.prepare_frame(:mixin => @name)
  raise Sass::SyntaxError.new("Undefined mixin '#{@name}'.") unless mixin = environment.mixin(@name)
  raise Sass::SyntaxError.new(<<END.gsub("\n", "")) if mixin.args.size < @args.size
n #{@name} takes #{mixin.args.size} argument#{'s' if mixin.args.size != 1}
 #{@args.size} #{@args.size == 1 ? 'was' : 'were'} passed.
  environment = mixin.args.zip(@args).
    inject(Sass::Environment.new(mixin.environment)) do |env, ((var, default), value)|
    env.set_local_var(var.name,
      if value
        value.perform(environment)
      elsif default
        val = default.perform(env)
        if default.context == :equals && val.is_a?(Sass::Script::String)
          val = Sass::Script::String.new(val.value)
        end
        val
      end)
    raise Sass::SyntaxError.new("Mixin #{@name} is missing parameter #{var.inspect}.") unless env.var(var.name)
    env
  end
  self.children = mixin.tree.map {|c| c.perform(environment)}.flatten
rescue Sass::SyntaxError => e
  if original_env # Don't add backtrace info if this is an @include loop
    e.modify_backtrace(:mixin => @name, :line => @line)
    e.add_backtrace(:line => @line)
  end
  raise e
ensure
  original_env.pop_frame if original_env
end