+Interval
+Paper_column::get_interface_extent (Grob *column, SCM iface, Axis a)
+{
+ Interval extent = Interval (0, 0);
+ extract_grob_set (column, "elements", elts);
+
+ for (vsize i = 0; i < elts.size (); i++)
+ if (elts[i]->internal_has_interface (iface))
+ extent.unite (robust_relative_extent (elts[i], elts[i], a));
+
+ return extent;
+}
+
+/*
+ Print a:
+ - vertical line,
+ - the rank number,
+ - rank moment,
+ - blue arrow representing ideal distance,
+ - red arrow representing minimum distance
+ to aid debugging. To turn this on, simply add
+ \override Score.PaperColumn #'stencil = #ly:paper-column::print
+ \override Score.NonMusicalPaperColumn #'stencil = #ly:paper-column::print
+ to your score.
+ Also, as of 2013-10-16 there's a switch in Frescobaldi that turns this on.
+*/
+MAKE_DOCUMENTED_SCHEME_CALLBACK (Paper_column, print, 1,
+ "Optional stencil for @code{PaperColumn} or"
+ "@code{NonMusicalPaperColumn}.\n"
+ "Draws the @code{rank number} of each column,"
+ " its moment in time, a blue arrow showing the"
+ " ideal distance, and a red arrow showing the"
+ " minimum distance between columns.");