code: def is_ref(variant, exclude_alleles=None):
      """Returns true if variant is a reference record.
  
      Variant protos can encode sites that aren't actually mutations in the
      sample. For example, the record ref='A', alt='.' indicates that there is
      no mutation present (i.e., alt is the missing value).
  
      Args:
        variant: nucleus.genomics.v1.Variant.
        exclude_alleles: list(str). The alleles in this list will be ignored.
  
      Returns:
        True if there are no actual alternate alleles.
      """
      relevant_alts = _non_excluded_alts(
          variant.alternate_bases, exclude_alleles
      )
      return not relevant_alts
  
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 16 for calling a global function with example usage: is_ref(variant, exclude_alleles) and returns: unknown