As for assumption of risk, courts have recognized that this defense overlaps that of contributory negligence to some extent and in fact is made up of at least two distinct defenses. 'To simplify greatly, it has been observed . . . that in one kind of situation, to wit, where a plaintiff unreasonably undertakes to encounter a specific known risk imposed by a defendant's negligence, plaintiff's conduct, although he may encounter that risk in a prudent manner, is in reality a form of contributory negligence . . . . Other kinds of situations within the doctrine of assumption of risk are those, for example, where plaintiff is held to agree to relieve defendant of an obligation of reasonable conduct toward him. Such a situation would not involve contributory negligence, but rather a reduction of defendant's duty of care.' (Grey v. Fibreboard Paper Products Co. (1966) 65 Cal.2d 240, 245-246 [53 Cal.Rptr. 545, 418 P.2d 153]; see also Fonseca v. County of Orange (1972) 28 Cal.App.3d 361, 368-369 [104 Cal.Rptr. 566]; see generally, 4 Witkin, Summary of Cal. Law, Torts, § 723, pp. 3013-3014; 2 Harper & James, The Law of Torts, supra, § 21.1, pp. 1162-1168; cf. Prosser, ...