Remark
19
[00S](Solved on 2022-10-11) Note that β\(β xβ A, π\)β is true if \(A\) is the empty set; this is consistent with what was discussed in the exercise [016]. This has though a striking consequence: the implication
\[ (β xβ A, π ) \Rightarrow (\exists xβ A, π) \]
is always valid when \(A\) is a non-empty set, but is instead false when \(A=\emptyset \).