/*! Return true if \p a includes \p b. */
friend pure bool includes(const sphere<T,N>& a,
const box<T,N>& b) {
const auto n = b.corner_count();
vec<T,N> c[n]; b.read_corners(c);
return std::all_of(c,c+n, [a](vec<T,N>& p) { return a.includes(p); });
}
