Forbid mixing clocks (#2670)

Closes #2365

Specifically:
* Forbids noClock between different base-clocks.
* Forbids system of equations between different sub-clocks.

Co-authored-by: Henrik Tidefelt <henrikt@wolfram.com>

chapters/synchronous.tex

@@ -778,7 +778,7 @@ \subsection{Sub-clock conversion operators}\label{sub-clock-conversion-operators
 \\ \hline
 \lstinline!noClock(u)!
 &
-The clock of \lstinline!y = noClock(u)! is always inferred. At every tick
+The clock of \lstinline!y = noClock(u)! is always inferred, and \lstinline!u! must be part of the same base-clock as \lstinline!y!. At every tick
 of the clock of \lstinline!y!, the operator returns the value of \lstinline!u! from the last
 tick of the clock of \lstinline!u!. If \lstinline!noClock(u)! is called before the
 first tick of the clock of \lstinline!u!, the start value of \lstinline!u! is returned.\\ \hline
@@ -1013,6 +1013,9 @@ \subsection{Sub-clock Partitioning}\label{sub-clock-partitioning}
 \textless{}E\textsubscript{ij}, V\textsubscript{ij}\textgreater{},
 according to sub-clocks.
 
+The resulting sets of equations and variables shall be possible to solve separately,
+meaning that systems of equations cannot involve different sub-clocks.
+
 It can be noted that:
 
 $E_{ij} \bigcap E_{kl} = \emptyset~ \forall i\ne{}k, j\ne{}l$