(a) A provision of a will or trust that if given effect would reduce or eliminate the interest of any beneficiary of such will or trust who initiates or participates in an action to contest the validity of such will or trust or to set aside or vary the terms of such will or trust shall be enforceable.
(b) The provisions of subsection (a) of this section shall have no application to:
(1) Any action brought by the trustee of a trust or the personal representative under a will;
(2) Any action in which the beneficiary is determined by the court to have prevailed substantially;
(3) Any agreement among beneficiaries of the will or trust in settlement of a dispute relating to such will or trust;
(4) Any action to determine whether a proposed or pending motion, petition, or other proceeding constitutes a contest within the meaning of a no-contest provision described in subsection (a) of this section; or
(5) Any action brought by a beneficiary under a will or trust instrument for a construction or interpretation of such will or trust instrument.”