input: 
  from typing import List
  
  def calculate_rrna_percent(htseq_counts_file: str, gtf_file: str) -> float:
      """ Calculate the percentage of transcripts mapping to rRNA in the given HTseq_counts file.
      
      Takes as input an HTseq_counts file and a GTF file. It will extract all the identifiers of
      genes/transcripts mapping to the rRNA and MT genes, count reads matching the rRNA identifiers,
      and calculate the percentage of reads that map to rRNA.
      
      >>> calculate_rrna_percent('C17_3_S20_Aligned.out.srt.bam_htseq_combined_counts.txt', '/path/to/GTF/file')
      15.7
      """
  
output:     
      rrna_ids = set()
      mt_ids = set()
      
      with open(gtf_file, 'r') as gtf:
          for line in gtf:
              if line.startswith('#'):
                  continue
              fields = line.strip().split('\t')
              if fields[2] == 'gene':
                  gene_info = fields[8]
                  if 'gene_biotype "rRNA"' in gene_info:
                      rrna_ids.add(gene_info.split(';')[0].split()[1].strip('"'))
                  elif 'gene_biotype "Mt_rRNA"' in gene_info:
                      mt_ids.add(gene_info.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()
              count = int(count)
              total_count += count
              if gene_id in rrna_ids or gene_id in mt_ids:
                  rrna_count += count
      
      return (rrna_count / total_count) * 100
  