+ *
+ * "single_valued" is set if the current inverse schedule (which may or may
+ * not be stored in "executed") is known to be single valued, specifically
+ * an inverse schedule that was not (appeared not to be) single valued
+ * is extended to a single valued inverse schedule. This is mainly used
+ * to avoid an infinite recursion when we fail to detect later on that
+ * the extended inverse schedule is single valued.