problem with interaction of plotyy/clf/delete

[Ben Abbott]

As deleting one axis of plotyy group results in the other axis being  
deleted, the clf() function results in an attempt to delete an axis a  
second time.

	demo plotyy
	clf

	error: delete: invalid graphics object (= -15.2239)
	error: called from:
	error:   /Users/bpabbott/Development/mercurial/octave-3.1.51/scripts/ 
miscellaneous/delete.m at line 40, column 1
	error:   /Users/bpabbott/Development/mercurial/octave-3.1.51/scripts/ 
plot/clf.m at line 78, column 1

How should this be resolved?

Shall delete() or __go_delete__ be modified ?

As the existence of the parent is already verified by __go_delete__ it  
seems that it would be appropriate the check that a call back hasn't  
deleted one of the figure's children.

I took a shot at changing graphics.cc to fix the problem, but it  
persists (my c++ skills are practically non-existent). I expect a  
proper solution is trivial, but still beyond me :-(

Ben