4. Graded Problem (Page limit: 1 sheet; 2 sides) Suppose you have been shown a new programming language that allows you to express the following: Any single symbol in the English alphabet set {A, B,...,Z} is expressible. These are considered strings of length 1. A special expression e also exists that denotes the empty string. Given some two expressions that can express some two subsets S and T of strings, you can form an expression that expresses its union SU T, its concatenation S.T-xy | xES,y E T!. Finally, if S is 4. Graded Problem (Page limit: 1 sheet; 2 sides) Suppose you have been shown a new programming language that allows you to express the following: Any single symbol in the English alphabet set {A, B,...,Z} is expressible. These are considered strings of length 1. A special expression e also exists that denotes the empty string. Given some two expressions that can express some two subsets S and T of strings, you can form an expression that expresses its union SU T, its concatenation S.T-xy | xES,y E T!. Finally, if S is