-grob is set to #t, that part of all objects in lower
-layers which falls under the extent of the grob is
-whited out. Here the TimeSignature whites out the
-Tie but not the StaffSymbol.
+grob is set to a number or #t, or if the 'whiteout-box
+property is set to #t, that part of all objects
+in lower layers which falls under the extent of
+the grob's whiteout area is whited out. Here the
+TimeSignature whites out the Tie but not the StaffSymbol.