code: def merge_vcf_files(infiles, ref_seqs, outfile, threads=1):
      '''infiles: list of input VCF file to be merge.
      outfile: name of output VCF file.
      threads: number of input files to read in parallel'''
      vars_dict = vcf_file_read.vcf_files_to_dict_of_vars(infiles, ref_seqs, threads=threads)
      _dict_of_vars_to_vcf_file(vars_dict, outfilef merge_vcf_files(infiles, ref_seqs, outfile, threads=1):
      '''infiles: list of input VCF file to be merge.
      outfile: name of output VCF file.
      threads: number of input files to read in parallel'''
      vars_dict = vcf_file_read.vcf_files_to_dict_of_vars(infiles, ref_seqs, threads=threads)
      _dict_of_vars_to_vcf_file(vars_dict, outfile)
docs: infiles: list of input VCF file to be merge.
      outfile: name of output VCF file.
      threads: number of input files to read in parallel