code: def _count_titv(single_stats):
      titv_counts = {"Transition": 0, "Transversion": 0}
      titv_counts["Transition"] = sum(
          [v.is_transition for v in single_stats]
      )
      titv_counts["Transversion"] = sum(
          [v.is_transversion for v in single_stats]
      )
      return titv_counts
  
imports: import collections
  import itertools
  import math
  import numpy
  from third_party.nucleus.util import variant_utils
  from third_party.nucleus.util import variantcall_utils
  from deepvariant import vcf_stats_vis
label_desc: Use image node_id 17 for calling a global function with example usage: _count_titv(single_stats) and returns: titv_counts