# The negative negation normal form (NNF).
	#
	# This method is used to implement `nnf`.
	# Basically, it returns the NNF of `~self`.
	#
	# ~~~
	# var a = new LVar("a")
	# var b = new LVar("b")
	# assert (a&b).nnnf.to_s == "(~a | ~b)"
	# ~~~
	#
	# Subclasses implements it by using De Morgan's laws to push negation inwards.
	fun nnnf: LExpr do return ~self