Rationals are Everywhere Dense in Sorgenfrey Line

Theorem
$\Q$ is dense in Sorgenfrey line.

Proof
Let $T = \left({\R, \tau}\right)$ be the Sorgenfrey line.

Define $\mathcal B = \left\{{\left[{x\,.\,.\,y}\right): x, y \in \R}\right\}$

By definition of Sorgenfrey line:
 * $\mathcal B$ is basis of $T$.

By definition of subset:
 * $\Q^- \subseteq \R$

where $\Q^-$ denotes the topological closure of $\Q$ in $T$.

By definition of set equality to prove the equality: $\Q^- = \R$, it should be show
 * $\R \subseteq \Q^-$

Let $x \in \R$.

By Characterization of Closure by Basis it suffices to prove that
 * $\forall U \in \mathcal B: x \in U \implies U \cap \Q \ne \varnothing$

Let $U \in \mathcal B$.

By definition of $\mathcal B$:
 * $\exists y, z \in \R: U = \left[{y\,.\,.\,z}\right)$

Assume
 * $x \in U$

By definition of half-open real interval:
 * $y \le x < z$

By Between two Real Numbers exists Rational Number:
 * $\exists q \in \Q: y < q < z$

By definition of half-open real interval:
 * $q \in U$

Thus by definitions of intersection and empty set
 * $U \cap \Q \ne \varnothing$