class_docstr: A container for the nodes and labelled edges of a dependency structure.
docstr: Adds an arc from the node specified by head_address to the
  node specified by the mod address.
docstr_tok: ["Adds","an","arc","from","the","node","specified","by","head_address","to","the","node","specified","by","the","mod","address","."]
code: def add_arc(self, head_address, mod_address):
      """
      Adds an arc from the node specified by head_address to the
      node specified by the mod address.
      """
      relation = self.nodes[mod_address]["rel"]
      self.nodes[head_address]["deps"].setdefault(relation, [])
      self.nodes[head_address]["deps"][relation].append(mod_address)
  
imports: import subprocess
  import warnings
  from collections import defaultdict
  from itertools import chain
  from pprint import pformat
  from nltk.internals import find_binary
  from nltk.tree import Tree
label_desc: Use image node_id 4 for calling the DependencyGraph obj's underlying member method code with example usage: obj.add_arc(head_address, mod_address) without return types