Add example from the half-reification section into the handling of undefined expressions

chapters/2_background.tex

@@ -491,7 +491,26 @@ both contexts.
 \label{subsec:back-mzn-partial}
 
 Some expressions in the \cmls\ do not always have a well-defined result.
-Examples of such expressions in \minizinc\ are:
+
+\begin{example}\label{ex:back-undef}
+  Consider the following ``complex constraint''
+
+  \begin{mzn}
+    constraint i <= 4 -> a[i] * x >= 6;
+  \end{mzn}
+
+  which requires that if \mzninline{i} takes a value less or equal to four, then
+  the value in the \texttt{i}\(^{th}\) position of array \mzninline{a}
+  multiplied by \mzninline{x} must be at least 6.
+
+  Suppose the array \texttt{a} has index set \mzninline{1..5}, but \texttt{i}
+  takes the value \texttt{7}. The constraint \mzninline{element(i, a, t1)} will
+  fail and no solution will be found. However, intuitively if \mzninline{i = 7}
+  the constraint should be trivially true.
+\end{example}
+
+Part of the semantics of a \cmls\ is the choice as to how to treat these partial
+functions. Examples of such expressions in \minizinc\ are:
 
 \begin{itemize}
   \item Division (or modulus) when the divisor is zero: