is a string, then only the matching UID is signed.  If it
        is a list of strings, then all matching UIDs are signed.  Note
        that a case-sensitive exact string comparison is done.

        EXPIRES_IN specifies the expiration time of the signature in
        seconds.  If EXPIRES_IN is False, the signature does not
        expire.

        Keyword arguments:
        uids         -- user ids to sign, see above (default: sign all)
        expires_in   -- validity period of the signature in seconds
                                               (default: do not expire)
        local        -- create a local, non-exportable signature
                                               (default: False)

        Raises:
        GPGMEError   -- as signaled by the underlying library

        r