Because a “plan” includes an “employee welfare benefit plan,” and because welfare plans offer benefits that do not “vest” (at least insofar as ERISA is concerned), Congress’ use of the word “plan” in Section 510 all but forecloses the argument that Section 510’s interference clause applies only to “vested” rights. Had Congress intended to confine Section 510’s protection to “vested” rights, it could have easily substituted the term “pension plan” for “plan,” or the term “nonforfeitable right” for “any right.” But Section 510 draws no distinction between those rights that “vest” under ERISA and those that do not.