The invention relates to a compound of formula 1in which R1, X1, X2, X3, X4, R4, Y1, Y2 and Y3 are as defined in claim 1. In order to achieve the object, according to this invention, an imaging optical system is formed from a single-element lens, and the sectional shapes of the incident and exit surfaces in the main scanning direction are non-arcuated, and the power in the sub scanning direction substantially concentrates on the exit surface. Certificate discovery identifies certificates and associated information including network locations, methods of access, applications of use and non-use, and may produce logs and reports.