866. The portion of income to which the first paragraph of section 865 refers is the portion that is not deemed under that paragraph to be income of an employee other than the particular employee and that may reasonably be considered, having regard to the circumstances and terms of the trust arrangement, as being included in(a) an amount included under section 859 in computing the income of the particular employee; or
(b) the amount by which the aggregate of every capital gain of the trust that is deemed to be a capital gain of the particular employee under section 860, exceeds the aggregate of every capital loss of the trust that is deemed to be a capital loss of the particular employee under that section.