code: def concatenate_fastas(output_fna_clustered,
                         output_fna_failures,
                         output_concat_filepath):
      
  
      output_fp = open(output_concat_filepath, "w")
  
      for label, seq in parse_fasta(open(output_fna_clustered, "U")):
          output_fp.write(">%s\n%s\n" % (label, seq))
      for label, seq in parse_fasta(open(output_fna_failures, "U")):
          output_fp.write(">%s\n%s\n" % (label, seq))
  
      return output_concat_filepath
docs: Concatenates two input fastas, writes to output_concat_filepath
  
      output_fna_clustered: fasta of successful ref clusters
      output_fna_failures: de novo fasta of cluster failures
      output_concat_filepath: path to write combined fastas to