Describe a new concept "simple connector" that is not balanced. (mode…

…lica#3356)

* Describe a new concept "simple connector" that is not balanced.
Proposal to resolve modelica#3347
Closes modelica#3347

Co-authored-by: Elena Shmoylova <eshmoylova@users.noreply.github.com>

chapters/connectors.tex

@@ -616,12 +616,17 @@ \section{Restrictions of Connections and Connectors}\label{restrictions-of-conne
 
 \subsection{Balancing Restriction and Size of Connectors}\label{balancing-restriction-and-size-of-connectors}
 
-For each non-partial non-expandable connector class the number of flow variables shall be equal to the number of variables that are neither \lstinline!parameter!, \lstinline!constant!, \lstinline!input!, \lstinline!output!, \lstinline!stream! nor \lstinline!flow!.
+For each non-partial non-simple non-expandable connector class the number of flow variables shall be equal to the number of variables that are neither \lstinline!parameter!, \lstinline!constant!, \lstinline!input!, \lstinline!output!, \lstinline!stream! nor \lstinline!flow!.
 The \emph{number of variables} is the number of all elements in the connector class after expanding all records and arrays to a set of scalars of primitive types.
 The number of variables of an overdetermined type or record class (see \cref{overconstrained-equation-operators-for-connection-graphs}) is the size of the output argument of the corresponding \lstinline!equalityConstraint!() function.
+A simple connector class is not expandable, has some time-varying variables, and has neither \lstinline!input!, \lstinline!output!, \lstinline!stream! nor \lstinline!flow! variables.
 \begin{nonnormative}
 Expandable connector classes are excluded from this, since their component declarations are only a form of constraint.
 \end{nonnormative}
+A component of a simple connector class must be declared as \lstinline!input!, \lstinline!output!, or \lstinline!protected!.
+\begin{nonnormative}
+A simple connector class is thus always unbalanced, but since it is used with causality or not visible to the outside it does not unbalance the model.
+\end{nonnormative}
 
 \begin{example}
 \begin{lstlisting}[language=modelica]