The function takes a string input and returns a greeting or farewell message based on the input.

[CODE]:
def function(input):
if input == "hello":
return "Hello, world!"
elif input == "goodbye":
return "Goodbye, world!"
else:
return "Hello again, world!"

Of course, this is just one possible implementation of the function, and there may be other ways to write it. Can you think of any other ways to implement this function based on the given behavior?