docs: def backwardeuler(dfun, xzero, timerange, timestep):
      '''Backward Euler method integration. This function wraps BackwardEuler.
  
          :param dfun:
              Derivative function of the system.
              The differential system arranged as a series of first-order
              equations: \dot{X} = dfun(t, x)
          :param xzero:
              The initial condition of the system.
          :param vzero:
              The initial condition of first derivative of the system.
          :param timerange:
              The start and end times as (starttime, endtime).
          :param timestep:
             The timestep.
          :param convergencethreshold:
              Each step requires an iterative solution of an implicit equation.
              This is the threshold of convergence.
          :param maxiterations:
              Maximum iterations of the implicit equation before raising
              an exception.
          :returns: t, x:
              as lists.
      '''
      return zip(*list(BackwardEuler(dfun, xzero, timerange, timestep)))