What is the test for denying a motion to substitute counsel for a defendant's attorney?

1 In passing, I note the following. The majority make the statement, which purports to derive ultimately from People v. Marsden (1970) 2 Cal.3d 118, 84 Cal.Rptr. 156, 465 P.2d 44, that "Denial of the motion [for substitution of appointed counsel] is not an abuse of discretion unless the defendant has shown that a failure to replace the appointed attorney would 'substantially impair' the defendant's right to assistance of counsel." (Maj. opn., ante, at p. 43 of 285 Cal.Rptr., at p. 1285 of 814 P.2d.)