效绵县 Informal Comments -(2) Basic idea is to use "variables"to stand for sets of strings (i.e.,languages) These variables are defined recursively,in terms of one another. Recursive rules "productions")involve only concatenation. Alternative rules for a variable allow union. .5Informal Comments – (2) ◼ Basic idea is to use “variables” to stand for sets of strings (i.e., languages). ◼ These variables are defined recursively, in terms of one another. ◼ Recursive rules (“productions”) involve only concatenation. ◼ Alternative rules for a variable allow union. 5