code: def build_fname(self, prefix, postfix, fmt, export_dir=None):
          
          if hasattr(postfix, 'sm_lt_path'):  # is a realization
              fname = '%s-rlz-%03d.%s' % (prefix, postfix.ordinal, fmt)
          else:
              fname = prefix + ('-%s' % postfix if postfix else '') + '.' + fmt
          return self.export_path(fname, export_dir)
docs: Build a file name from a realization, by using prefix and extension.
  
          :param prefix: the prefix to use
          :param postfix: the postfix to use (can be a realization object)
          :param fmt: the extension ('csv', 'xml', etc)
          :param export_dir: export directory (if None use .export_dir)
          :returns: relative pathname including the extension