Constrain use of 'delay' with zero delay time

chapters/operatorsandexpressions.tex

@@ -824,9 +824,9 @@ \subsection{Derivative and Special Purpose Operators with Function Syntax}\label
 \begin{semantics}
 Evaluates to \lstinline!$\mathit{expr}$(time - $\mathit{delayTime}$)! for $\text{\lstinline!time!} > \text{\lstinline!time.start!} + \mathit{delayTime}$ and \lstinline!$\mathit{expr}$(time.start)! for $\text{\lstinline!time!} \leq \text{\lstinline!time.start!} + \mathit{delayTime}$.
 The arguments, i.e., $\mathit{expr}$, $\mathit{delayTime}$ and $\mathit{delayMax}$, need to be subtypes of \lstinline!Real!.
-$\mathit{delayMax}$ needs to be additionally a parameter expression.
-The following relation shall hold: $0 \leq \mathit{delayTime} \leq \mathit{delayMax}$, otherwise an error occurs.
-If $\mathit{delayMax}$ is not supplied in the argument list, $\mathit{delayTime}$ needs to be a parameter expression.
+$\mathit{delayMax}$ shall be a parameter expression.
+If $\mathit{delayMax}$ is not supplied in the argument list, $\mathit{delayTime}$ is required to be evaluable.
+It is required that $0 \leq \mathit{delayTime} \leq \mathit{delayMax}$, with $0 = \mathit{delayTime}$ only being allowed when $\mathit{delayMax}$ is not provided (that is, when $\mathit{delayTime}$ is required to be evaluable).
 The operator is not allowed inside \lstinline!function! classes.
 For non-scalar arguments the function is vectorized according to \cref{vectorized-calls-of-functions}.
 For further details, see \cref{delay}.
@@ -943,9 +943,9 @@ \subsubsection{delay}\label{delay}
 For real-time simulation where fixed step size integrators are used, this information is sufficient to allocate the necessary storage for the internal buffer before the simulation starts.
 For variable step size integrators, the buffer size is dynamic during integration.
 
-In principle, \lstinline!delay! could break algebraic loops.
-For simplicity, this is not supported because the minimum delay time has to be given as additional argument to be fixed at compile time.
-Furthermore, the maximum step size of the integrator is limited by this minimum delay time in order to avoid extrapolation in the delay buffer.
+In order to avoid extrapolation in the delay buffer, the maximum step size of the integrator is limited by the delay time, making delay times close to zero a potential source of poor simulation performance.
+
+In the form where $\mathit{delayTime}$ is required to be evaluable, a tool can decide to evaluate $\mathit{delayTime}$ only when the value would be zero, and introduce a runtime check that it remains non-zero otherwise.
 \end{nonnormative}
 
 \subsubsection{spatialDistribution}\label{spatialdistribution}