class_docstr: Defined by J. Welles Wilder, Jr. in 1978 in his book *"New Concepts in
  Technical Trading Systems"* for the RSI
  
  SAR stands for *Stop and Reverse* and the indicator was meant as a signal
  for entry (and reverse)
  
  How to select the 1st signal is left unspecified in the book and the
  increase/decrease of bars
  
  See:
    - https://en.wikipedia.org/wiki/Parabolic_SAR
    - http://stockcharts.com/school/doku.php?id=chart_school:technical_indicators:parabolic_sar
code: def nextstart(self):
      if self._status:  # some states have been calculated
          self.next()  # delegate
          return
  
      # Prepare a status holding array, for current and previous lengths
      self._status = [_SarStatus(), _SarStatus()]
  
      # Start by looking if price has gone up/down (close) in the 2nd day to
      # get an *entry* signal and configure the values as they would have
      # been in the previous trend, including a sar value which is
      # immediately invalidated in next, which reverses and sets the trend to
      # the actual up/down value calculated with the close
      # Put the 4 status variables in a Status holder
      plenidx = (len(self) - 1) % 2  # previous length index (0 or 1)
      status = self._status[plenidx]
  
      # Calculate the status for previous length
      status.sar = (self.data.high[0] + self.data.low[0]) / 2.0
  
      status.af = self.p.af
      if self.data.close[0] >= self.data.close[-1]:  # uptrend
          status.tr = not True  # uptrend when reversed
          status.ep = self.data.low[-1]  # ep from prev trend
      else:
          status.tr = not False  # downtrend when reversed
          status.ep = self.data.high[-1]  # ep from prev trend
  
      # With the fake prev trend in place and a sar which will be invalidated
      # go to next to get the calculation done
      self.next()
  
file_path: backtrader/backtrader/indicators/psar.py
imports: from __future__ import absolute_import, division, print_function, unicode_literals
  from .None import PeriodN
label_desc: Use image node_id 2 for calling the ParabolicSAR obj's underlying member method code with example usage: obj.nextstart() without return types