+MAKE_SCHEME_CALLBACK (Self_alignment_interface, centered_on_note_columns, 1);
+SCM
+Self_alignment_interface::centered_on_note_columns (SCM smob)
+{
+ Item *it = unsmob_item (smob)->get_column ();
+ if (!it)
+ return scm_from_double (0.0);
+
+ extract_grob_set (it, "elements", elts);
+ vector<Grob *> ncs;
+ Interval centers;
+ for (vsize i = 0; i < elts.size (); i++)
+ if (Note_column::has_interface (elts[i]))
+ centers.add_point (scm_to_double (centered_on_object (elts[i], X_AXIS)));
+
+ if (centers.is_empty ())
+ return scm_from_double (0.0);
+
+ return scm_from_double (centers.center ());
+}
+