[ODE] quickstep vs faststep

[Thomas Yeh]

I am trying to understand the main differences between the current and the
old iterative solving method (quickstep vs faststep).

>From the documentation, I thought that the iterative method solves every
joint separately. I can see this clearly in the code for stepfast.cpp, but
quickstep.cpp's code looks different.

Can anyone point me to a more detailed description of quickstep.cpp or
give me a high-level algorithmic description?