- ERROR? ModalDecisionTrees.entropy([1,2],2) == ModalDecisionTrees.entropy([0,2],2)
- Question: without pruning, must purity always increase?
	entl=ModalDecisionTrees.entropy([0,40],40)*40                        
	entr=ModalDecisionTrees.entropy([1,3],4)*4                           
	ent=ModalDecisionTrees.entropy([1,43],44)                         
	newent = (entl + entr)/44
	purity = -ent
	newpurity = -newent
