Hello, thanks for your reply!
Actually, I am defining a function with two methods:
# method 1
FDEsolver(F, tSpan, y0, β, par...)
method 2
FDEsolver(F, tSpan, y0, β, JacobF, par...)
But because they both accept a vararg as last positional parameter, the method with fewer parameters is getting overwritten by the second method. In other words, when I use method 1 with the vararg par..., instead of using method 1 it uses method 2 with JacobF in place of par....
So I thought maybe I could define Jacob in both methods and skip it when I want to use method 1. I hope it helps to understand a little bit.