code: def _serialize_lnk(lnk):
      
      s = ""
      if lnk is not None:
          s = '<'
          if lnk.type == Lnk.CHARSPAN:
              cfrom, cto = lnk.data
              s += ''.join([str(cfrom), ':', str(cto)])
          elif lnk.type == Lnk.CHARTSPAN:
              cfrom, cto = lnk.data
              s += ''.join([str(cfrom), '#', str(cto)])
          elif lnk.type == Lnk.TOKENS:
              s += ' '.join([str(t) for t in lnk.data])
          elif lnk.type == Lnk.EDGE:
              s += ''.join(['@', str(lnk.data)])
          s += '>'
      return s
docs: Serialize a predication lnk to surface form into the SimpleMRS
         encoding.