What is the test for credibility of a witness in a sexual assault case?

examination]; see also Evid. Code, 780 ["jury may consider in determining the credibility of a witness any matter that has any tendency in reason to prove or disprove . . . [] . . . [] (i) The existence of nonexistence of any fact testified to by him"]; People v. Gallagher (1893) 100 Cal. 466, 476 ["It was not the intention of the legislature to give to a defendant the opportunity of making any statement upon his direct examination which he might choose, in reference to the issue before the court, and to preclude the prosecution from showing out of his own mouth that such statement is false"].)