21600. This part shall apply to property passing by will through a
decedent's estate or by a trust, as defined in Section 82, that
becomes irrevocable only on the death of the settlor.
21601. (a) For purposes of this part, "decedent's testamentary
instruments" means the decedent's will or revocable trust.
(b) "Estate" as used in this part shall include a decedent's
probate estate and all property held in any revocable trust that
becomes irrevocable on the death of the decedent.