init init_aforgroup (
		n_ids: Collection[Object], # Should be Collection[TId]
		n_kwin: nullable TKwin,
		n_expr: nullable AExpr
	)
	do
		self.n_ids.unsafe_add_all(n_ids)
		_n_kwin = n_kwin.as(not null)
		n_kwin.parent = self
		_n_expr = n_expr.as(not null)
		n_expr.parent = self
	end