code: def mpi_tag_key(a):
      """Break MPI ties by using the variable tag - prefer lower tags first."""
      if isinstance(a.op, (MPISend, MPIRecv, MPIRecvWait, MPISendWait)):
          return a.op.tag
      else:
          return 0
  
imports: from __future__ import absolute_import, print_function, division
  import numpy
  from theano import gof
  from theano.gof import Constant, Generic, Op
  from theano.gof.sched import key_to_cmp
  from theano.tensor import tensor
  import theano
label_desc: Use image node_id 7 for calling a global function with example usage: mpi_tag_key(a) and returns: a, int