code: def calculate_error(self):
          
          self.numvars.error = 0.
          fluxes = self.sequences.fluxes
          for flux in fluxes.numerics:
              results = getattr(fluxes.fastaccess, '_%s_results' % flux.name)
              diff = (results[self.numvars.idx_method] -
                      results[self.numvars.idx_method-1])
              self.numvars.error = max(self.numvars.error,
                                       numpy.max(numpy.abs(diff)))
docs: Estimate the numerical error based on the fluxes calculated
          by the current and the last method.
  
          >>> from hydpy.models.test_v1 import *
          >>> parameterstep()
          >>> model.numvars.idx_method = 2
          >>> results = numpy.asarray(fluxes.fastaccess._q_results)
          >>> results[:4] = 0., 3., 4., 0.
          >>> model.calculate_error()
          >>> from hydpy import round_
          >>> round_(model.numvars.error)
          1.0