docs: def euler(dfun, xzero, timerange, timestep):
      '''Euler method integration. This function wraps the Euler class.
  
          :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 timerange:
              the start and end times as (starttime, endtime)
          :param timestep:
              the timestep
          :returns: t, x:
              as lists
      '''
      return zip(*list(Euler(dfun, xzero, timerange, timestep)))