+
+void
+Item::handle_prebroken_dependencies ()
+{
+ Grob::handle_prebroken_dependencies ();
+
+ /*
+ Can't do this earlier, because try_visibility_lambda () might set
+ the elt property transparent, which would then be copied.
+ */
+ if (!Item::break_visible (this))
+ suicide ();
+}
+
+bool
+Item::break_visible (Grob *g)
+{
+ Item *it = dynamic_cast<Item*> (g);
+ SCM vis = g->get_property ("break-visibility");
+ if (scm_is_vector (vis))
+ return to_boolean (scm_c_vector_ref (vis, it->break_status_dir () + 1));
+ return true;
+}
+
+bool
+Item::pure_is_visible (int start, int end) const
+{
+ SCM vis = get_property ("break-visibility");
+ if (scm_is_vector (vis))
+ {
+ int pos = 1;
+ int pc_rank = Paper_column::get_rank (get_column ());
+ if (pc_rank == start)
+ pos = 2;
+ else if (pc_rank == end)
+ pos = 0;
+ return to_boolean (scm_vector_ref (vis, scm_from_int (pos)));
+ }
+ return true;
+}
+
+Interval_t<int>
+Item::spanned_rank_interval () const
+{
+ int c = get_column ()->get_rank ();
+ return Interval_t<int> (c, c);
+}
+
+Interval_t<Moment>
+spanned_time_interval (Item *l, Item *r)
+{
+ Drul_array<Item*> bounds (l, r);
+ Interval_t<Moment> iv;
+
+ Direction d = LEFT;
+ do
+ {
+ if (bounds[d] && bounds[d]->get_column ())
+ iv[d] = robust_scm2moment (bounds[d]->get_column ()->get_property ("when"),
+ iv[d]);
+ }
+ while (flip (&d) != LEFT);
+
+ do
+ {
+ if (!bounds[d] || !bounds[d]->get_column ())
+ iv[d] = iv[-d];
+ }
+ while (flip (&d) != LEFT);
+
+
+ return iv;
+}
+
+
+void
+Item::derived_mark () const
+{
+ if (broken_to_drul_[LEFT])
+ scm_gc_mark (broken_to_drul_[LEFT]->self_scm ());
+ if (broken_to_drul_[RIGHT])
+ scm_gc_mark (broken_to_drul_[RIGHT]->self_scm ());
+}
+
+Item *
+unsmob_item (SCM s)
+{
+ return dynamic_cast<Item *> (unsmob_grob (s));
+}
+
+Interval
+Item::pure_height (Grob *g, int start, int end)
+{
+ if (cached_pure_height_valid_)
+ return cached_pure_height_ + pure_relative_y_coordinate (g, start, end);
+
+ cached_pure_height_ = Grob::pure_height (this, start, end);
+ cached_pure_height_valid_ = true;
+ return cached_pure_height_ + pure_relative_y_coordinate (g, start, end);
+}
+
+bool
+Item::less (Grob * const &g1, Grob * const &g2)
+{
+ return dynamic_cast<Item*> (g1)->get_column ()->get_rank () < dynamic_cast<Item*> (g2)->get_column ()->get_rank ();
+}
+
+ADD_INTERFACE (Item,
+ "Grobs can be distinguished in their role in the horizontal"
+ " spacing. Many grobs define constraints on the spacing by"
+ " their sizes, for example, note heads, clefs, stems, and all"
+ " other symbols with a fixed shape. These grobs form a"
+ " subtype called @code{Item}.\n"
+ "\n"
+ "Some items need special treatment for line breaking. For"
+ " example, a clef is normally only printed at the start of a"
+ " line (i.e., after a line break). To model this,"
+ " @q{breakable} items (clef, key signature, bar lines, etc.)"
+ " are copied twice. Then we have three versions of each"
+ " breakable item: one version if there is no line break, one"
+ " version that is printed before the line break (at the end of"
+ " a system), and one version that is printed after the line"
+ " break.\n"
+ "\n"
+ "Whether these versions are visible and take up space is"
+ " determined by the outcome of the @code{break-visibility}"
+ " grob property, which is a function taking a direction"
+ " (@code{-1}, @code{0} or@tie{}@code{1}) as an argument. It"
+ " returns a cons of booleans, signifying whether this grob"
+ " should be transparent and have no extent.\n"
+ "\n"
+ "The following variables for @code{break-visibility} are"
+ " predefined:\n"
+ "@example\n"
+ " grob will show: before no after\n"
+ " break break break\n"
+ " all-invisible no no no\n"
+ " begin-of-line-visible no no yes\n"
+ " end-of-line-visible yes no no\n"
+ " all-visible yes yes yes\n"
+ " begin-of-line-invisible yes yes no\n"
+ " end-of-line-invisible no yes yes\n"
+ " center-invisible yes no yes\n"
+ "@end example",
+
+ /* properties */
+ "break-visibility "
+ "extra-spacing-height "
+ "extra-spacing-width "
+ "non-musical "
+ );