40 ILCS 5/19-204
The term "employe" under this Division shall include all persons in the employ of the public library board prior to July 8, 1955, receiving a stipulated salary per annum; all persons in the employ of the board of trustees of such Public Library Employes' Pension Fund, prior to July 8, 1955, receiving a stipulated salary; all persons who are contributors to this fund and have contributed to this fund for a period of at least 1 year and who may be transferred to any other department of the city or board of education by reason of the fact that the functions performed by such persons have been transferred from the jurisdiction of the library board to that of such other department of the city or board of education; and this Division shall apply only to those employes who voluntarily accept and agree to comply with its provisions.
Any employe, a part of whose salary may be set apart to provide for such fund may be released from the necessity of making further payments to said fund by filing a written notice of his or her desire to withdraw from complying with the provisions of this Division with the board of trustees hereinafter mentioned, which resignation shall operate and go into effect immediately upon its receipt by said board of trustees.
40 ILCS 5/19-204