These routines provides access to variable history at lagged times.
The lagged time \(t\) must not be less than \(t_0\), nor
should it be greater than the current time of gradient calculation.
The routine cannot be directly called by a user, and will only work
during the integration process as triggered by the dde routine.
pastvalue(t)
pastgradient(t)Access history at time t.
Vector of variable history at time t.