[ODE] Rewinding time / backward time stepping

[Thomas Harte]

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I am trying to greatly simplify the mesh on mesh collision problem by being able to 
move backward in time. The process will only ever be to have a particular point, to move 
forward from it and then possibly discard that forward motion and return to the original 
setting. I am wondering about the best way to achieve this.

This is the full definition of dWorldStep from the docs :

"void dWorldStep(dWorldID, dReal stepsize);

	Step the world."

So, am I allowed negative step sizes? Assuming I am, does anyone care to speculate on 
how much error I would get through stepping forward t units in time, then stepping back 
the same amount?

Even if I am allowed negative step sizes, I appreciate that using them is probably a bad 
idea when I can simply backup the original state, and then if necessary restore it. Which 
leads me to wondering about the best implementation of that. So, some questions :

Is there already a means for backing up a state, then restoring it? An analogue to the 
state and matrix stacks in OpenGL, for example.

If there is not, is there a way of polling for every body in a world, allowing me to write 
entire world state backup and restore functions as a black box?

Or, at the end of the day, am I looking at finding my own method of keeping track of 
every body in a world (which, of course, I almost certainly already have, but this will be 
application specific) and iterating through them in my own way? If so them I am likely to 
write my own functions to encapsulate all world functions and will almost certainly end up 
with unnecessary memory usage as I keep a list of all bodies in parallel to the ODE 
internals. Which is the main reason I want to avoid this if possible.

-Thomas
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