def perform(environment)
      _perform(environment)
    rescue Sass::SyntaxError => e
      e.modify_backtrace(:line => line)
      raise e
    end
def perform(environment)
      _perform(environment)
    rescue Sass::SyntaxError => e
      e.modify_backtrace(:line => line)
      raise e
    end
Evaluates the node.

 \{#perform} shouldn't be overridden directly;
 instead, override \{#\_perform}.

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