What is the test for establishing that a party relying on the presumption that a deceased person exercised ordinary care for his own concerns was negligent?

This presumption is not dispelled by evidence produced by the opposing party standing alone or by that of plaintiffs' witnesses unless their testimony is wholly irreconcilable with the presumption and not the product of mistake or inadvertence. Gigliotti v. Nunes, 45 Cal.2d 85, 92-93, 286 P.2d 809, 813: 'As expressed in Scott v. Burke (1952), 39 Cal.2d 388, 394, 247 P.2d 313, it is settled law that where alleged negligent acts and conduct of a decedent are at issue before the court and the 'testimony respecting such acts and conduct necessarily must be produced by witnesses other than the deceased, * * * an instruction that the deceased is presumed to have exercised ordinary care for his own concerns is * * * proper' except that if the fact proved by uncontradicted testimony produced by the party seeking to invoke the presumption, 'under circumstances which afford no indication that the testimony is the product of mistake or inadvertence * * * is wholly irreconcilable with the presumption * * * the latter is dispelled and disappears from the case.' [236 Cal.App.2d 559] '* * * the rule is established that if such person be deceased or unable to testify by reason of loss of memory, the fact that other witnesses for the parties testify fully as to the acts and conduct of the allegedly negligent person does not deprive the party relying on the presumption of the benefit thereof unless the testimony which he himself produces 'under circumstances which afford no indication that the testimony is the product of mistake or inadvertence * * * is wholly irreconcilable with the presumption sought to be invoked.''