code: def is_singleton(variant):
      """Returns True iff the variant has exactly one non-ref VariantCall."""
      non_ref_count = 0
      for c in variant.calls:
          if variantcall_utils.has_variation(c):
              non_ref_count += 1
              if non_ref_count > 1:
                  return False
      return non_ref_count == 1
  
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 47 for calling a global function with example usage: is_singleton(variant) and returns: unknown, False