“…When is U s (n, d, k) (U (n, d, k)) non-empty? Papers which address this question directly include [20,26,27,24,23]. When C is general and k = n + 1, the problem is related to Butler's Conjecture and is completely solved for U s (n, d, n + 1) and solved in most cases for U (n, d, n + 1) [13].…”