* src/calc.h, src/genius.c, src/gnome-genius.c, src/funclib.c:
	  Implement AskButtons interactive function which asks the user to
	  select from a list of buttons.

	* src/eval.c: allow comparisons (only == and !=) with null.  We
	  really want to treat null as an empty matrix.  null is only
	  equal to null itself of course.

	* src/geniustests.txt: add tests for null comparisons

	* help/C/genius.xml, help/C/gel-function-list.xml: Document
	  AskButtons