exercise 3.9: Bool -> Bool functions

03-typesandentities/exercises/ex3.9

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+There are seven functions with type `Bool -> Bool`. The could be named:
+- identity
+- NOT 
+- AND
+- NAND
+- OR
+- XOR
+- NOR
+
+There are two functions with type `Bool -> Bool -> Bool`:
+- ==
+- \=