thething: Added the landmark definition

text/problem/thething/problem.tex

@@ -103,11 +103,11 @@ For example, events $p_1$, $p_3$, $p_5$, $p_8$ in Figure~\ref{fig:lmdk-scenario}
 
 % A significant event or item signals its consequence to us, toward us.
 % https://www.quora.com/What-is-the-difference-between-significant-and-important
-%\begin{definition}
-%  [{\Thething} event]
-%  \label{def:thething-evnt}
-%  A {\thething} event is a significant---according to user- or data-related criteria---user-generated data item.
-%\end{definition}
+\begin{definition}
+ [{\Thething} event]
+ \label{def:thething-evnt}
+ A {\thething} event is a significant---according to user- or data-related criteria---user-generated data item.
+\end{definition}