\begin{definition}[Definition:Real Interval/Unbounded Closed]
There are two '''unbounded closed intervals''' involving a real number $a \in \R$, defined as:
{{begin-eqn}}
{{eqn | l = \hointr a \to
      | o = :=
      | r = \set {x \in \R: a \le x}
}}
{{eqn | l = \hointl \gets a
      | o = :=
      | r = \set {x \in \R: x \le a}
}}
{{end-eqn}}
\end{definition}