Specify inverse.

chapters/functions.tex

@@ -1515,10 +1515,9 @@ \section{Declaring Inverses of Functions}\label{declaring-inverses-of-functions}
 More than one inverse can be defined within the same \lstinline!inverse! annotation.
 Several inverses are separated by commas.
 
-\begin{nonnormative}
-The inverse requires that for all valid values of the input arguments of \lstinline!$f_2$($\ldots$, y, $\ldots$)! and $u_k$ being calculated as \lstinline!$u_k$ := $f_2$($\ldots$, y, $\ldots$)! implies
-the equality \lstinline!y = $f_1$($\ldots$, $u_k$, $\ldots$)! up to a certain precision.
-\end{nonnormative}
+The function in the \lstinline!inverse! annotation must be an actual inverse.
+This requires that for all valid values of the input arguments of \lstinline!$f_2$($\ldots$, y, $\ldots$)! and $u_k$ being calculated as \lstinline!$u_k$ := $f_2$($\ldots$, y, $\ldots$)! implies
+the equality \lstinline!y = $f_1$($\ldots$, $u_k$, $\ldots$)! is satisfied up to a certain precision.
 
 Function $f_1$ can have any number and types of formal parameters with and without default value.
 The restriction is that the \emph{number of unknown variables} (see \cref{balanced-models}) in the output formal parameter of both $f_1$ and $f_2$ must be the same and that $f_2$ should have a union of output and formal parameters that is the same or a sub-set of that union for $f_1$, but the order of the formal parameters may be permuted.