... A \ {a 3, a 6}, a 21∈ A \ {a 2, a 14, a 16}, a 22∈ A \ {a 2, a 5, a 7, a 11, a 14},and a 23∈ A \ {a 2, a 5, a 7, a 11, a 14, a 16}.We distinguish four cases.Case 1: a 18= ... with Lemma 2.4 shows that a 1, a 2, a 3, a 4, a 5, a 6, a 7, a 8, a 9, a 10, a 11, a 12, a 13, a 14, a 15, a 16, a 17, a 18, a 19are pairwise distinct and we are done.We are now ... Lemma 2.4, we can verify that a 1, a 2, . . . , a 16are pairwise distinct, and we also have a 17= a 1, . . . , a 13, a 15, a 16; a 18= a 1, . . . , a 5, a 7, . . . , a 10, a 16, a 17.If...