code: def __getValueAtImpl(self, timestamp):
      ret = None
      if self.windowFull():
          a, b = lsreg(self._timestamps.data(), self.getValues())
          ret = a * timestamp + b
      return ret
  
imports: from pyalgotrade import technical
  from pyalgotrade.utils import collections
  from pyalgotrade.utils import dt
  import numpy
  from scipy import stats
label_desc: Use image node_id 3 for calling the LeastSquaresRegressionWindow obj's underlying member method code with example usage: obj.__getValueAtImpl(timestamp) and returns: ret