code: def add_hcons(self, hcons):
          
          # (hi, relation, lo)
          _vars = self._vars
          _hcons = self._hcons
          for hc in hcons:
              try:
                  if not isinstance(hc, HandleConstraint):
                      hc = HandleConstraint(*hc)
              except TypeError:
                  raise XmrsError('Invalid HCONS data: {}'.format(repr(hc)))
  
              hi = hc.hi
              lo = hc.lo
              if hi in _hcons:
                  raise XmrsError(
                      'Handle constraint already exists for hole %s.' % hi
                  )
              _hcons[hi] = hc
              # the following should also ensure lo and hi are in _vars
              if 'hcrefs' not in _vars[lo]:
                  _vars[lo]['hcrefs'] = []
              for role, refs in _vars[hi]['refs'].items():
                  for nodeid in refs:
                      _vars[lo]['hcrefs'].append((nodeid, role, hi))
docs: Incorporate the list of HandleConstraints given by *hcons*.