PROB §695: (a) Unless the power to revoke is in the creating instrument or exists pursuant to Section 15400, the creation of a power of appointment is irrevocable.

(a) Unless the power to revoke is in the creating instrument or exists pursuant to Section 15400, the creation of a power of appointment is irrevocable. (b) Unless made expressly irrevocable by the creating instrument or the instrument of exercise, an exercise of a power of appointment is revocable if the power to revoke exists pursuant to Section 15400 or so long as the interest in the appointive property, whether present or future, has not been transferred or become distributable pursuant to the appointment. (c) Unless the power to revoke is reserved in the instrument releasing the power, a release of a power of appointment is irrevocable.