def _perform(environment)
      res = ""
      before = @before.perform(environment)
      res << before.value
      mid = @mid.perform(environment)
      res << (mid.is_a?(Sass::Script::Value::String) ? mid.value : mid.to_s(:quote => :none))
      res << @after.perform(environment).value
      opts(Sass::Script::Value::String.new(res, before.type))
    end
def _perform(environment)
      res = ""
      before = @before.perform(environment)
      res << before.value
      mid = @mid.perform(environment)
      res << (mid.is_a?(Sass::Script::Value::String) ? mid.value : mid.to_s(:quote => :none))
      res << @after.perform(environment).value
      opts(Sass::Script::Value::String.new(res, before.type))
    end
Evaluates the interpolation.

 @param environment [Sass::Environment] The environment in which to evaluate the SassScript
 @return [Sass::Script::Value::String]
   The SassScript string that is the value of the interpolation