# File lib/sass/script/interpolation.rb, line 59
    def _perform(environment)
      res = ""
      res << @before.perform(environment).to_s if @before
      res << " " if @before && @whitespace_before
      val = @mid.perform(environment)
      res << (val.is_a?(Sass::Script::String) ? val.value : val.to_s)
      res << " " if @after && @whitespace_after
      res << @after.perform(environment).to_s if @after
      opts(Sass::Script::String.new(res))
    end