code: def is_gvcf(variant):
      """Returns true if variant encodes a standard gVCF reference block.
  
      This means in practice that variant has a single alternate allele that is the
      canonical gVCF allele vcf_constants.GVCF_ALT_ALLELE.
  
      Args:
        variant: nucleus.genomics.v1.Variant.
  
      Returns:
        Boolean. True if variant is a gVCF record, False otherwise.
      """
      return variant.alternate_bases == [vcf_constants.GVCF_ALT_ALLELE]
  
imports: from __future__ import absolute_import
  from __future__ import division
  from __future__ import print_function
  import collections
  import enum
  import itertools
  import six
  from third_party.nucleus.protos import variants_pb2
  from third_party.nucleus.util import ranges
  from third_party.nucleus.util import variantcall_utils
  from third_party.nucleus.util import vcf_constants
label_desc: Use image node_id 32 for calling a global function with example usage: is_gvcf(variant) and returns: unknown