class_docstr: This class is responsible for scanning over historical data and analyzing returns before
  and after the events.
  
  :param predicate: A :class:`Predicate` subclass responsible for identifying events.
  :type predicate: :class:`Predicate`.
  :param lookBack: The number of bars before the event to analyze. Must be > 0.
  :type lookBack: int.
  :param lookForward: The number of bars after the event to analyze. Must be > 0.
  :type lookForward: int.
code: def __addPastReturns(self, instrument, event):
      begin = (event.getLookBack() + 1) * -1
      for t in xrange(begin, 0):
          try:
              ret = self.__rets[instrument][t]
              if ret is not None:
                  event.setValue(t + 1, ret)
          except IndexError:
              pass
  
imports: import numpy
  import matplotlib.pyplot
  from six.moves import xrange
  from pyalgotrade.technical import roc
  from pyalgotrade import dispatcher
label_desc: Use image node_id 2 for calling the Profiler obj's underlying member method code with example usage: obj.__addPastReturns(instrument, event) without return types