code: def save_nff(self, dest, _strategy="auto"):
      
      if _strategy not in ("auto", "write", "mmap"):
          raise TValueError("Invalid parameter _strategy: only 'write' / 'mmap' "
                            "/ 'auto' are allowed")
  
      dest = os.path.expanduser(dest)
      if not os.path.exists(dest):
          os.makedirs(dest)
  
      self.materialize()
      mins = self.min().to_list()
      maxs = self.max().to_list()
  
      metafile = os.path.join(dest, "_meta.nff")
      with _builtin_open(metafile, "w", encoding="utf-8") as out:
          out.write("# NFF2\n")
          out.write("# nrows = %d\n" % self.nrows)
          out.write('filename,stype,meta,colname,min,max\n')
          l = len(str(self.ncols))
          for i in range(self.ncols):
              filename = "c%0*d" % (l, i + 1)
              colname = self.names[i].replace('"', '""')
              stype = self.stypes[i]
              if stype == dt.stype.obj64:
                  dtwarn("Column %r of type obj64 was not saved" % self.names[i])
                  continue
              smin = _stringify(mins[i][0])
              smax = _stringify(maxs[i][0])
              out.write('%s,%s,,"%s",%s,%s\n'
                        % (filename, stype.code, colname, smin, smax))
              filename = os.path.join(dest, filename)
              core._column_save_to_disk(self, i, filename, _strategy)
docs: Save Frame in binary NFF/Jay format.
  
      :param dest: destination where the Frame should be saved.
      :param _strategy: one of "mmap", "write" or "auto"