code: def count_mismatches_before_variant(reference_prefix, cdna_prefix):
      
      if len(reference_prefix) != len(cdna_prefix):
          raise ValueError(
              "Expected reference prefix '%s' to be same length as %s" % (
                  reference_prefix, cdna_prefix))
      return sum(xi != yi for (xi, yi) in zip(reference_prefix, cdna_prefix))
docs: Computes the number of mismatching nucleotides between two cDNA sequences before a variant
      locus.
  
      Parameters
      ----------
      reference_prefix : str
          cDNA sequence of a reference transcript before a variant locus
  
      cdna_prefix : str
          cDNA sequence detected from RNAseq before a variant locus