code: def get_ref_allele_frequency(variant):
      """Gets REF allele frequency for a Variant proto."""
      sum_alt_frequency = 0
      for alt_idx, _ in enumerate(variant.alternate_bases):
          sum_alt_frequency += get_allele_frequency(variant, alt_idx)
      return 1 - sum_alt_frequency
  
file_path: deepvariant/deepvariant/allele_frequency.py
imports: import collections
  from typing import DefaultDict, Sequence, Optional
  from absl import logging
  from third_party.nucleus.io import vcf
  from third_party.nucleus.util import ranges
  from third_party.nucleus.util import variant_utils
  from deepvariant.protos import deepvariant_pb2
label_desc: Use image node_id 2 for calling a global function with example usage: get_ref_allele_frequency(variant) and returns: unknown