code: def _get_last_transverse_over_list(self, zxq):
          
          z_max = None
          x_of_interest = None
  
          for z, x, q in zxq:
              if z == None or z > z_max:
                  x_of_interest = x
  
          return x_of_interest
docs: Get transverse coord at highest z
  
          :param zx: A list where each element is (z, transverse, charge)