def add_icons(self, icons):
        """
        Incorporate the individual constraints given by *icons*.
        """
        _vars, _icons = self._vars, self._icons
        for ic in icons:
            try:
                if not isinstance(ic, IndividualConstraint):
                    ic = IndividualConstraint(*ic)
            except TypeError:
                raise XmrsError('Invalid ICONS data: {}'.format(repr(ic)))
            left = ic.left
            right = ic.right
            if left not in _icons:
                _icons[left] = []
            _icons[left].append(ic)
            # the following should also ensure left and right are in _vars
            if 'icrefs' not in _vars[right]:
                _vars[right]['icrefs'] = []
            _vars[right]['icrefs'].append(ic)
            _vars[left]
Incorporate the individual constraints given by *icons*.