void GoRegion::Find2FreeLibs(const GoChain* c1, const GoChain* c2,
                             SgPoint* lib1, SgPoint* lib2) const
{
    SgPointSet libs = Points() & c1->FreeLiberties() & c2->FreeLiberties();
    if (CHECK)
        SG_ASSERT(libs.MinSetSize(2));
    SgSetIterator it(libs);
    *lib1 = *it;
    ++it;
    *lib2 = *it;
}