code: def unify_walk(a, b, U):
      """
      NV(list1) == NV(list2) == NV(union(list1, list2))
  
      """
      opt = a.not_options.union(b.not_options)
      v = NotVariable("?", opt)
      return U.merge(v, a, b)
  
imports: from __future__ import absolute_import, print_function, division
  from copy import copy
  from functools import partial
  from theano.gof.utils import ANY_TYPE, comm_guard, FALL_THROUGH, iteritems
label_desc: Use image node_id 10 for calling a global function with example usage: unify_walk(a, b, U) and returns: U