code: def is_deletion(reference_bases, alternate_bases):
      
      # if multiple alts, it is unclear if we have a transition
      if len(alternate_bases) > 1:
          return False
  
      if is_indel(reference_bases, alternate_bases):
          # just one alt allele
          alt_allele = alternate_bases[0]
          if alt_allele is None:
              return True
          if len(reference_bases) > len(alt_allele):
              return True
          else:
              return False
      else:
          return False
docs: Return whether or not the INDEL is a deletion