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