+ /* find the closest scale-point, return the internal value of
+ * the prev/next scale-point (no wrap-around)
+ *
+ * If the given parameter is not en enum, the given val is returned.
+ *
+ * @param val internal (not interface) value
+ * @param prev if true, step to prev scale-point, otherwise next
+ * @return internal value, suitable for set_value()
+ */
+ float step_enum (float val, bool prev) const;
+