4.1 Give context-free grammars generating the following sets S a) The set of palindromes (strings that read the same forward as backward) over alphabet {a, b). b) The set of all strings of balanced parentheses, i.e., each left parenthesis has a matching right parenthesis and pairs of matching parentheses are properly nested. *c) The set of all strings over alphabet {a, b) with exactly twice as many a's as b's. d) The set of all strings over alphabet sa, b. , + ., (,)E, that are well-formed regular expressions over alphabet {a, b). Note that we must distinguish between c as the empty string and as a symbol in a regular expression. We use e in the latter case. e) The set of all strings over alphabet (a, b not of the form ww for some string w. f) labc li j or j +k