+ /*
+ * Offset of the tail of @dst. This needs to be moved out of the way
+ * to make space for the runs to be copied from @src, i.e. the first
+ * run of the tail of @dst.
+ * Nominally, @tail equals @loc + 1, i.e. location, skipping the
+ * replaced run. However, if @right, then one of @dst's runs is
+ * already merged into @src.
+ */
+ tail = loc + right + 1;
+ /*
+ * First run after the @src runs that have been inserted, i.e. where
+ * the tail of @dst needs to be moved to.
+ * Nominally, @marker equals @loc + @ssize, i.e. location + number of
+ * runs in @src. However, if @left, then the first run in @src has
+ * been merged with one in @dst.
+ */
+ marker = loc + ssize - left;