If a trustee determines that an allocation between principal and income required by section 18, 19, 20, 21 or 24 is insubstantial, the trustee may allocate the entire amount to principal unless 1 of the circumstances described in subsection (c) of section 4 applies to the allocation. This power may be exercised by a cotrustee in the circumstances described in subsection (d) of said section 4 and may be released for the reasons and in the manner described in subsection (e) of said section 4. An allocation is presumed to be insubstantial if:
Mass. Gen. Laws ch. 203D, § 17