code: def backward(self, indexes, gy):
      x = self.get_retained_inputs()[0]
      return (
          (2 / numpy.pi) ** 0.5 / erfcx.erfcx(-x / 2**0.5) * gy[0],
      )
  
file_path: chainer/chainer/functions/math/log_ndtr.py
imports: import numpy
  from chainer.backends import cuda
  from chainer import function_node
  from chainer.functions.math import erfcx
  from chainer import utils
  from chainer.utils import type_check
label_desc: Use image node_id 5 for calling the LogNdtr obj's underlying member method code with example usage: obj.backward(indexes, gy) and returns: unknown