code: def nav_to_vcf_dir(ftp, build):
      
      if build == 'b37':
          ftp.cwd(DIR_CLINVAR_VCF_B37)
      elif build == 'b38':
          ftp.cwd(DIR_CLINVAR_VCF_B38)
      else:
          raise IOError("Genome build not recognized.")
docs: Navigate an open ftplib.FTP to appropriate directory for ClinVar VCF files.
  
      Args:
          ftp:   (type: ftplib.FTP) an open connection to ftp.ncbi.nlm.nih.gov
          build: (type: string) genome build, either 'b37' or 'b38'