+ (remove-layer ,integer? "The @code{Keep_alive_together_engraver}
+removes all @code{VerticalAxisGroup} grobs with a @code{remove-layer}
+larger than the smallest retained @code{remove-layer}. Set to
+@code{#f} to make a layer invisible to the
+@code{Keep_alive_together_engraver}, set to @code{'()} to have it not
+participate in the layering decisions.")