"hich may be optionally substituted??? represented by X1 include the same groups as those exemplified for the substituent in the term of the above-mentioned ???cyclic hydrocarbon group which may be optionally substituted??? represented by R1, and the like, and are preferably lower alkyl (for example, C1-6 alkyl such as methyl, ethyl, propyl and the like), lower alkenyl (for example, C2-6 alkenyl su" . . . .