What is the test for a defendant to prove that snow and ice on a parking lot constitutes an unusual danger?

The summation of Mr. Justice Schroeder in that case might well apply to the present case. He said at p. 316: "… In our opinion it is extremely doubtful, having regard to the season of the year, that the presence of snow and ice on the appellant's parking lot constituted an unusual danger. In any event, it is abundantly plain on the evidence that the plaintiff was fully aware of that condition and of all its implications. Under the rule in Indermaur v. Dames, supra, the duty resting upon the defendant occupier of property is to prevent damage from unusual danger which he knows or ought to know. It is a valid defence if he has clearly and sufficiently warned the invitee of the existence of that unusual danger. In this case a specific warning to the plaintiff would have been wholly superfluous, since the plaintiff on his own admission was completely aware of the condition that existed. Moreover the defendant has clearly established that the plaintiff freely and voluntarily with full knowledge of the nature and extent of the risk he ran impliedly agreed to incur it. Thus the defendant is entitled to invoke and rely upon the maxim volenti non fit injuria. While it may be sufficient for the defendant to show that the plaintiff incurred the risk sciens, it has nevertheless established that he also incurred it volens."