Consider Delay differential Equations

[mattfidler]

Thank you for the clarification. I noticed the ODE solvers, and it worked well with stiff and non-stiff equations implemented in nlmixr.
To comment for robust DDE solver, NONMEM ADVAN 16 (Delay DE) (to be released later this year) is implementing this FORTRAN solver RADAR5.

Originally posted by @Mutaz94 in #161 (comment)