code: public Map<Var, VarTensor> getVarMarginalsIndexed() {
      	Map<Var, VarTensor> m = new HashMap<Var, VarTensor>();
      	for(VarTensor df : margs) {
      		assert df.getVars().size() == 1;
      		m.put(df.getVars().get(0), df);
      	}
      	return m;
      }
docs: Convenience wrapper around getVarMarginals().
  Does not memoize, so use sparingly.