Using generic types as function return values

Within an implementation block that has generic type parameters, we can use those parameter names as part of function return values, too.

Here is an example function that uses generic type parameter names in its return data type:

    fn get_ref(&self, key: K) -> Result<&V, String> {
        if key == self.key {
            return Ok(&self.value);
        }
        else if key < self.key {
            match self.lesser {
                None => {
                    return Err("No such key".to_string());
                }
                Some(ref lesser) => {
                    return lesser.get_ref(key);
                }
            }
        }
        else {
            match self.greater {
                None => {
                    return Err("No such key".to_string());
                }
  ...