+
+SCM
+Item::do_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 ());
+ return SCM_EOL;
+}
+
+Item*
+unsmob_item (SCM s )
+{
+ return dynamic_cast<Item*> (unsmob_grob (s));
+}
+
+
+
+ADD_INTERFACE (Item,
+ "item-interface",
+ "\n"
+ "\n"
+ "Grobs can be distinguished in their role in the horizontal spacing.\n"
+ "Many grobs define constraints on the spacing by their sizes. For\n"
+ "example, note heads, clefs, stems, and all other symbols with a fixed\n"
+ "shape. These grobs form a subtype called @code{Item}.\n"
+ "\n"
+ "\n"
+ "Some items need special treatment for line breaking. For example, a\n"
+ "clef is normally only printed at the start of a line (i.e. after a\n"
+ "line break). To model this, `breakable' items (clef, key signature,\n"
+ "bar lines, etc.) are copied twice. Then we have three versions of each\n"
+ "breakable item: one version if there is no line break, one version\n"
+ "that is printed before the line break (at the end of a system), one\n"
+ "version that is printed after the line break.\n"
+ "\n"
+ "Whether these versions are visible and take up space, is determined by\n"
+ "the outcome of the @code{break-visibility}. This grob property is a\n"
+ "function taking a direction (-1, 0 or 1) as argument. It returns a\n"
+ "cons of booleans, signifying whether this grob should be transparent\n"
+ "and have no extent.\n"
+ "\n"
+ "The following variables for 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"
+ "@end example\n"
+ ,
+ "no-spacing-rods break-visibility breakable")