If the amount of income required to be distributed currently to beneficiaries exceeds the distributable net income of the trust (as defined in section 643(a)), each beneficiary includes in his gross income an amount equivalent to his proportionate share of such distributable net income. Thus, if beneficiary A is to receive two-thirds of the trust income and B is to receive one-third, and the income required to be distributed currently is $99,000, A will receive $66,000 and B, $33,000. However, if the distributable net income, as determined under section 643(a) is only $90,000, A will include two-thirds ($60,000) of that sum in his gross income, and B will include one-third ($30,000) in his gross income. See §§1.652(b)-1 and 1.652(b)-2, however, for amounts which are not includible in the gross income of a beneficiary because of their tax-exempt character.