Section 40 ILCS 5/9-131 - Prior service annuity credits(a) The sum to be credited for prior service annuity in the case of any present employee described in subdivision (a) of Section 9-109 shall be the entire sum credited for such purposes.(b) The sum to be credited for prior service annuity in the case of any present employee described in subdivision (b) of Section 9-109 shall be the sum credited for such purpose less the excess which would have accumulated under this Article from contributions by the employee after he attained age 65 if such contributions had been made from the effective date to the date of withdrawal with interest at the effective rate to the date of his withdrawal, over the amounts actually contributed for such purpose with like interest computed to such date of withdrawal; provided that the sum so computed shall be less than the sum credited for prior service annuity under the foregoing provisions of this Article. If the sum so computed shall be equal to or greater than the sum credited for prior service annuity as aforesaid, such employee shall not be entitled to prior service annuity.