Consider a data type for logical expressions data Expr = Var String Or [Expr] And [Expr] -- A variable with given name | | | Not Expr deriving (Eq, Show) Write a function collect :: Expr -> ([String], Int), which col- lects a list of unique variable names in an expression, and the total number of times a "Not" appears. You may write helper functions if you wish. For example > expr = And [Var "a", Or [Var "a", Var "b"], Not (Var "c")] > collect expr ( ["a", "b", "c"], 1)