code: def only_call(variant):
      """Ensures the Variant has exactly one VariantCall, and returns it.
  
      Args:
        variant: nucleus.genomics.v1.Variant. The variant of interest.
  
      Returns:
        The single nucleus.genomics.v1.VariantCall in the variant.
  
      Raises:
        ValueError: Not exactly one VariantCall is in the variant.
      """
      if len(variant.calls) != 1:
          raise ValueError(
              "Expected exactly one VariantCall in {}".format(variant)
          )
      return variant.calls[0]
  
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 1 for calling a global function with example usage: only_call(variant) and returns: variant