[Halld-offline] getting generated information

[Mark M. Ito]

  Folks,

General question:

I have an hdgeant-generated event with a single particle. Here is 
snippet of the hddm:

<physicsEvent eventNo="1" runNo="1">
<reaction type="0" weight="0">
<vertex>
<product decayVertex="0" id="1" mech="0" parentid="0" pdgtype="0" 
type="pi+">
<momentum E="0" px="0" py="0" pz="100" />
</product>
<origin t="0" vx="-100" vy="-100" vz="170" />
</vertex>
</reaction>

How do I get access to the position of the vertex, i. e., (vx, vy, vz) 
where "v" stands for vertex, from within JANA?

   -- Mark