input: def calculate_rRNA_percent(htseq_counts_file: str, gtf_file: str) -> str:
      """Calculates the percent of transcripts mapping to rRNA.
      
      >>> calculate_rRNA_percent("C17_3_S20_Aligned.out.srt.bam_htseq_combined_counts.txt", "/Users/malwina/Documents/CTR-Data/genomes/Mus_musculus/mm10/Mus_musculus.GRCm38.84.gtf")
      'Summary output file:		C17_3_S20_Aligned.out.srt.bam_htseq_combined_counts_rRNAmtRNACounts.txt'
      """
output:     rRNA_features = set()
      with open(gtf_file, 'r') as gtf:
          for line in gtf:
              if line.startswith('#'):
                  continue
              fields = line.strip().split('\t')
              if fields[2] == 'rRNA':
                  rRNA_features.add(fields[8].split(';')[0].split()[1].strip('"'))
  
      rRNA_count = 0
      total_count = 0
      with open(htseq_counts_file, 'r') as htseq:
          for line in htseq:
              if line.startswith('__'):
                  continue
              gene_id, count = line.strip().split()
              total_count += int(count)
              if gene_id in rRNA_features:
                  rRNA_count += int(count)
  
      rRNA_percent = (rRNA_count / total_count) * 100
      output_file = f"{htseq_counts_file.rsplit('.', 1)[0]}_rRNAmtRNACounts.txt"
      with open(output_file, 'w') as out:
          out.write(f"rRNA Count: {rRNA_count}\n")
          out.write(f"Total Count: {total_count}\n")
          out.write(f"rRNA Percent: {rRNA_percent:.2f}%\n")
  
      return f"Summary output file:\t\t{output_file}"