The `once` keyword

Property definitions

	# The `once` keyword
	var n_kwonce: TKwonce is writable, noinit

src/parser/parser_nodes.nit:2132,2--2133,42

	redef fun n_kwonce=(node)
	do
		_n_kwonce = node
		node.parent = self
	end

src/parser/parser_prod.nit:3600,2--3604,4