Let the rationals $q_i\in\Bbb {Q}$ be enumerated by the naturals $i\in\Bbb {N}$
Base case {$q_1$}, complement $(-\infty,q_1)\cup (q_1,\infty)$ true
2nd case {$q_1$} $\cup$ {$q_2$}, complement $(-\infty,q_1)\cup (q_1,q_2) \cup (q_2,\infty)$ true
Inductive case $\bigcup_{i=1}^k$ {$q_i$} $\cup $ {$q_{k+1}$}, complement $(-\infty,q_1)\cup (\bigcup_{i=1}^k(q_i,q_{i+1})) \cup (q_{k+1},\infty)$ true
Induction to the naturals
$(-\infty,q_1)\cup (\bigcup_{i\in \Bbb {N}}(q_i,q_i+1))$
-> ???