+/**
+ * Compute the play time based on information of the given slot.
+ *
+ * \param slot_num The slot number (negative means: no slot).
+ *
+ * This computes a string of the form "0:07 [3:33] (3%/3:40)" using information
+ * from the status items received from para_server and the start time of the
+ * (first) writer of the given slot.
+ *
+ * It has to to take into account that probably the stream was not started at
+ * the beginning of the file, that the clock between the server and the client
+ * host may differ and that playback of the stream was delayed, e.g. because
+ * the prebuffer filter is used in the filter configuration of the given slot.
+ *
+ * If no writer is active in the given slot, or \a slot_num is negative
+ * (indicating that para_audiod runs in standby mode), an approximation based
+ * only on the status items is computed and the returned string is prefixed
+ * with "~".
+ *
+ * \return A string that must be freed by the caller.
+ */