code: private void zeroOutSigmaXt(final Vec x_t)
      {
          //Zero out temp store
         if(diagonalOnly && x_t.isSparse())//only these values will be non zero 
             for(IndexValue iv : x_t)
                 Sigma_xt.set(iv.getIndex(), 0.0);
         else
             Sigma_xt.zeroOut();
      }
docs: More than one escape point, makes sure to zero out {@link #Sigma_xt}
  using the input incase of sparseness
  @param x_t