Why is there no single variable resistor?

Problem description: 

The PLECS library contains variable resistors only in combination with inductors and capacitors. Why is there no single variable resistor in PLECS?


The Simulink/PLECS solvers require the system to be described by ordinary differential equations (ODEs) and are not capable of solving general differential algebraic equations (DAEs). Therefore, a variable or non-linear resistor must be modeled either as a voltage-controlled current source or as a current-controlled voltage source. To avoid algebraic loops, the sources must be decoupled by a state variable, hence the inductor or capacitor.