other, are hydrogen atoms or alkyl groups, and X is an anion; (d) those of the general formula ##STR35## wherein R1 is an alkyl group, R2 and R3 each, independently of the other, are hydrogen atoms or alkyl groups, and X is an anion; (e) those of the general formula ##STR36## wherein R1 is an alkyl group, R2 is a hydrogen atom or an alkyl group, and X is an anion; (f) those of the general formula