code: def nexmake(mdict, nlocus, dirs, mcmc_burnin, mcmc_ngen, mcmc_sample_freq):
      
      ## create matrix as a string
      max_name_len = max([len(i) for i in mdict])
      namestring = "{:<" + str(max_name_len+1) + "} {}\n"
      matrix = ""
      for i in mdict.items():
          matrix += namestring.format(i[0], i[1])
      
      ## write nexus block
      handle = os.path.join(dirs, "{}.nex".format(nlocus))
      with open(handle, 'w') as outnex:
          outnex.write(NEXBLOCK.format(**{
              "ntax": len(mdict), 
              "nchar": len(mdict.values()[0]), 
              "matrix": matrix,
              "ngen": mcmc_ngen, 
              "sfreq": mcmc_sample_freq, 
              "burnin": mcmc_burnin, 
              }))
docs: function that takes a dictionary mapping names to 
      sequences, and a locus number, and writes it as a NEXUS
      file with a mrbayes analysis block.