Vec(VecExpression<E> const& vec): elems(vec.size()) { for (size_t i=0; i != vec.size(); ++i) elems[i] = vec[i]; }
VecDifference(VecExpression<E1> const& u, VecExpression<E2> const& v) : _u(u), _v(v) { assert(u.size() == v.size()); }