step (Fix e)          = case step e of
                          Left e' -> Left (Fix e')
                          Right v -> Left (unfix v)
step (Fix e)          = case step e of
                          Left e' -> Left (Fix e')
                          Right v -> Left (unfix v)
step (Fix e)          = case step e of
                          Left e' -> Left (Fix e')
                          Right v -> Left (unfix v)