. . . . . "(3) A disulfone compound represented by the following formula (PAG5) and an iminosulfonate compound represented by the following formula (PAG6): ##STR11## wherein Ar3 and Ar4 are each a substituted or unsubstituted aryl group, R6 is a substituted or unsubstituted alkyl or aryl group, and A is a substituted or unsubstituted alkylene, alkenylene or arylene group." . .