The ???leaving group??? in the definition of L represents generally a group which leaves as a nucleophilic residue, and can be, for example, a halogen atom such as a fluorine atom, a chlorine atom, a bromine atom or an iodine atom; a lower alkanesulfonyloxy group such as a methanesulfonyloxy or ethenesulfonyloxy group; a halogeno lower alkanesulfonyloxy group such as a trifluoromethanesulfonyloxy