This is an improved implementation of a Negative Impedance
Converter with opAmps.
Note that the opAmp model has to take into account saturation of
the output.
Note that p.i + n.i <> 0 due to the active opAmp (with
implicit supply) inside!
If voltage v is prescribed, the current i can be unambiguously
determined.
If current i is prescribed, in the range between the zero crossings
the voltage v has 2 or 3 possible solutions.