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 prenext(self):
      if len(self) == 1:
          self._status = []  # empty status
          return  # not enough data to do anything
  
      elif len(self) == 2:
          self.nextstart()  # kickstart calculation
      else:
          self.next()  # regular calc
  
      self.lines.psar[0] = float("NaN")
  
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 1 for calling the ParabolicSAR obj's underlying member method code with example usage: obj.prenext() without return types