vent is a compound represented by the following formula (III) or (IV): ##STR26## wherein R1 and R2 each represents an alkyl group having 1 to 4 carbon atoms; and X is a hydrogen atom, a halogen atom, a methyl group, an ethyl group, a methoxy group, an ethoxy group or an aryl group; ##STR27## wherein R3 is ##STR28## wherein R31 and R32 each represents a hydrogen atom or an alkyl group, with the pro