def PrependUnique(self, delete_existing=0, **kw):
        kw = copy_non_reserved_keywords(kw)
        for key, val in kw.items():
            if SCons.Util.is_List(val):
                val = _delete_duplicates(val, not delete_existing)
            if key not in self._dict or self._dict[key] in ('', None):
                self._dict[key] = val
            elif SCons.Util.is_Dict(self._dict[key]) and \
                 SCons.Util.is_Dict(val):
                self._dict[key].update(val)
            elif SCons.Util.is_List(val):
                dk = self._dict[key]
                if not SCons.Util.is_List(dk):
                    dk = [dk]
                if delete_existing:
                    dk = [x for x in dk if x not in val]
                else:
                    val = [x for x in val if x not in dk]
                self._dict[key] = val + dk
            else:
                dk = self._dict[key]
                if SCons.Util.is_List(dk):
                    if delete_existing:
                        dk = [x for x in dk if x not in val]
                        self._dict[key] = [val] + dk
                    else:
                        if not val in dk:
                            self._dict[key] = [val] + dk
                else:
                    if delete_existing:
                        dk = [x for x in dk if x not in val]
                    self._dict[key] = val + dk
        self.scanner_map_delete(kw)