code: def exponential_log_prob(self, val):
      rate = self.parameters["rate"]
      result = tf.log(rate) - rate * val
      return result
  
imports: from __future__ import absolute_import
  from __future__ import division
  from __future__ import print_function
  import numpy
  import tensorflow
  from edward.models.random_variables import *
label_desc: Use image node_id 8 for calling a global function with example usage: exponential_log_prob(val) and returns: result