code: def step_forward(self):
          
          tenddict = self.compute()
          #  Total tendency is applied as an explicit forward timestep
          # (already accounting properly for order of operations in compute() )
          for varname, tend in tenddict.items():
              self.state[varname] += tend * self.timestep
          # Update all time counters for this and all subprocesses in the tree
          #  Also pass diagnostics up the process tree
          for name, proc, level in walk.walk_processes(self, ignoreFlag=True):
              if proc.time['active_now']:
                  proc._update_time()
docs: Updates state variables with computed tendencies.
  
          Calls the :func:`compute` method to get current tendencies for all
          process states. Multiplied with the timestep and added up to the state
          variables is updating all model states.
  
          :Example:
  
              ::
  
                  >>> import climlab
                  >>> model = climlab.EBM()
  
                  >>> # checking time step counter
                  >>> model.time['steps']
                  0
  
                  >>> # stepping the model forward
                  >>> model.step_forward()
  
                  >>> # step counter increased
                  >>> model.time['steps']
                  1