+ return SCM_UNSPECIFIED;
+}
+
+/* for each grob, find its upper and lower skylines. If the grob has
+ an empty extent, delete it from the list instead. If the extent is
+ non-empty but there is no skyline available (or pure is true), just
+ create a flat skyline from the bounding box */
+static void
+get_skylines (Grob *me,
+ vector<Grob*> *const elements,
+ Axis a,
+ bool pure, int start, int end,
+ vector<Skyline_pair> *const ret)
+{
+ /* each child's skyline was calculated according to the common refpoint of its
+ elements. Here we need all the skylines to be positioned with respect to
+ a single refpoint, so we need the common refpoint of the common refpoints
+ of the elements of the children */
+ vector<Grob*> child_refpoints;
+ for (vsize i = 0; i < elements->size (); i++)
+ {
+ Grob *elt = (*elements)[i];
+ Grob *child_common = unsmob_grob ((a == Y_AXIS)
+ ? elt->get_object ("X-common")
+ : elt->get_object ("Y-common"));
+
+ if (!child_common)
+ {
+ extract_grob_set (elt, "elements", child_elts);
+ child_common = common_refpoint_of_array (child_elts, elt, other_axis (a));
+ }
+
+ child_refpoints.push_back (child_common);
+ }