code: def add_edge(f, source, dest):
      writeln(f, "{0} -> {1};".format(source, dest))
  
imports: from __future__ import unicode_literals
  from contextlib import contextmanager
  import errno
  from functools import partial
  from io import BytesIO
  from subprocess import Popen, PIPE
  from networkx import topological_sort
  from six import iteritems
  from catalyst.pipeline.data import BoundColumn
  from catalyst.pipeline import Filter, Factor, Classifier, Term
  from catalyst.pipeline.term import AssetExists
label_desc: Use image node_id 15 for calling a global function with example usage: add_edge(f, source, dest) without return types