# File lib/sass/tree/mixin_node.rb, line 56
    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("Mixin \#{@name} takes \#{mixin.args.size} argument\#{'s' if mixin.args.size != 1}\n but \#{@args.size} \#{@args.size == 1 ? 'was' : 'were'} passed.\n".gsub("\n", "")) if mixin.args.size < @args.size
      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