struct df_ext_iterator : public df_iterator<T, SetTy, true> {
df_ext_iterator(const df_iterator<T, SetTy, true> &V)
: df_iterator<T, SetTy, true>(V) {}
struct df_ext_iterator : public df_iterator<T, SetTy, true> {
df_ext_iterator(const df_iterator<T, SetTy, true> &V)
: df_iterator<T, SetTy, true>(V) {}