\section{Possibly interesting Structures}
\begin{definition}

    An element of a rieg $x\in R$ is \emph{\loga} if the exponential function $y\mapsto x^y$ is injective. 
\end{definition}

\memo{Is the subobject classifier \loga? This is difficult question. For example, even if two sets have the same cardinality of powersets, we cannot prove those sets have same cardinality. In fact, it is independent from ZFC.}

\memo{endo of $(0, \infty)$}