code: def is_transition(allele1, allele2):
      """Is the pair of single bp alleles a transition?
  
      Args:
        allele1: A string of the first allele, must be 1 bp in length.
        allele2: A string of the second allele, must be 1 bp in length.
  
      Returns:
        True if allele1/allele2 are a transition SNP.
  
      Raises:
        ValueError: if allele1 and allele2 are equal or aren't 1 bp in length.
      """
      if allele1 == allele2:
          raise ValueError("Alleles must be unique:", allele1, allele2)
      if len(allele1) != 1:
          raise ValueError("Alleles must be 1 bp in length.", allele1)
      if len(allele2) != 1:
          raise ValueError("Alleles must be 1 bp in length.", allele2)
  
      alleles_set = {allele1, allele2}
      return any(alleles_set == x for x in [{"A", "G"}, {"C", "T"}])
  
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 18 for calling a global function with example usage: is_transition(allele1, allele2) and returns: any