[ODE] ERP for ball-joints

[Brian Allen]

Hello All--

I'm wanting to model the behavior (trajectory) of PD-controllers using
ball-joints and AMotors.  I'm running into a previously-discussed
problem:  there's no parameter for ERP on a per-joint basis.

I found the old patch for ODE 0.35 that was kindly posted to the list,
but before trying to get it working, I'm wondering why the amotor ERP
param is missing; especially since a parameter to set CFM is
available.  Is there some fundamental problem here?  Is there a reason
for the asymmetry?

Does anyone have any suggestions for another (stable) way to get
spring-like behavior from ball-joints?

Thanks for your help.
brian