Part-time (employment that is less than one-half the standard work load), irregular, seasonal, or temporary employment (employment for a definite period of less than four and 1/2 months or, for employment with an institution of higher education, the employment is for no more than one semester in a school year) is eligible only if combined with other employment with the same employer so that the combined employment qualifies as service eligible for membership, or if other employment with the same or another TRS-covered employer in itself qualifies as service eligible for membership.