The type of the problem depends on a parameter: if such a parameter is different
from zero, then it is a stiff system of 15 non-linear Ordinary Differential
Equations; instead if it is equal to zero, we have a Differential Algebraic
Equation of index 2, consisting of 11 differential and 4 algebraic equations.
The problem originates from electrical circuit analysis, and it describes
the behavior of a ring modulator.
A complete description of the problem could be found in
ringmod.pdf