Your comment helped me get this: function HALT(program, input): # This is a "magic" function for our proof - we assume it exists for all programs # I

Your comment helped me get this: function HALT(program, input): # This is a "magic" function for our proof - we assume it exists for all programs # In reality, this function cannot exist because of the paradox we're about to create # Returns True if `program(input)` halts, False if it runs forever ... # Program D: Creates the paradox function D(self): if HALT(D, self) == True: loop_forever() else: halt()