+ /** @param frame frame within the whole (untrimmed) content.
+ * @param frame_rate The frame rate of the audio (it may have been resampled).
+ * @return a fade coefficient for @ref length samples starting at an offset @frame within
+ * the content, or an empty vector if the given section has no fade.
+ */
+ std::vector<float> fade (AudioStreamPtr stream, Frame frame, Frame length, int frame_rate) const;