code: def allele_counts_dataframe(variant_and_allele_reads_generator):
      
      df_builder = DataFrameBuilder(
          AlleleCount,
          extra_column_fns={
              "gene": lambda variant, _: ";".join(variant.gene_names),
          })
      for variant, allele_reads in variant_and_allele_reads_generator:
          counts = count_alleles_at_variant_locus(variant, allele_reads)
          df_builder.add(variant, counts)
      return df_builder.to_dataframe()
docs: Creates a DataFrame containing number of reads supporting the
      ref vs. alt alleles for each variant.