Clarify that locally non-singular can combine to give a global singul…

…arity. (#2529)

Note that locally non-singular may combine to globally non-singular or globally singular;
and the latter is the exception that should be mentioned.
Closes #2510

chapters/classes.tex

@@ -1212,7 +1212,7 @@ \section{Balanced Models}\doublelabel{balanced-models}
 unknowns is structurally non-singular. However, this does not provide
 any guarantees for the global set of equations, and specific
 combinations of models that are locally non-singular may lead to a
-globally non-singular model.}{]}
+globally singular model.}{]}
 
 \emph{Example 3:}
 \begin{lstlisting}[language=modelica]