T17977.hs:31:1: warning: [GHC-62161] [-Wincomplete-patterns (in -Wextra)]
    Pattern match(es) are non-exhaustive
    In an equation for ‘f’:
        Patterns of type ‘SNat m’, ‘SNat n’, ‘SNat o’,
                         ‘R m n o’ not matched:
            SZ _ _ _
            (SS _) _ SZ _