code: def apply_preactivation(l, preact):
      if preact == "bnrelu":
          shortcut = l  # preserve identity mapping
          l = BNReLU("preact", l)
      else:
          shortcut = l
      return l, shortcut
  
file_path: tensorpack/examples/Saliency/resnet_model.py
imports: import tensorflow
  from tensorpack import tfv1
  from tensorpack.models import BatchNorm, BNReLU, Conv2D, FullyConnected, GlobalAvgPooling, MaxPooling
  from tensorpack.tfutils.argscope import argscope, get_arg_scope
label_desc: Use image node_id 3 for calling a global function with example usage: apply_preactivation(l, preact) and returns: l,  shortcut