code: def contigs_n_bases(contigs):
      """Returns the sum of all n_bases of contigs."""
      return sum(c.n_bases for c in contigs)
  
imports: from __future__ import absolute_import
  from __future__ import division
  from __future__ import print_function
  import collections
  import re
  from typing import Dict, Iterable, Optional, Sequence
  from absl import logging
  from etils import epath
  import intervaltree
  import six
  from third_party.nucleus.io import bed
  from third_party.nucleus.protos import position_pb2
  from third_party.nucleus.protos import range_pb2
  from third_party.nucleus.protos import reference_pb2
  from third_party.nucleus.protos import variants_pb2
label_desc: Use image node_id 12 for calling a global function with example usage: contigs_n_bases(contigs) and returns: sum