Yeah, I'm sure deftype does nothing.

  (deftype option (subtype)
    "The type of a value that is either nil or a value of SUBTYPE."
    `(or null ,subtype))

  (defun* get-latest-statement ((account account))
    "If any statements have been made on the ACCOUNT, returns the latest.

  Otherwise returns nil."
    (:returns (option statement))
    (car (last (account-statements account))))

I like defstar, it's a neat package.