void initialize() {
if(valid_) return;
f_.reset();
valid_ = (begin_ != end_)?
f_(begin_,end_,tok_):false;
}
void initialize(Base& b){
if(b.valid_) return;
func_.reset();
b.valid_ = (b.p_.first != b.p_.second)?
func_(b.p_.first,b.p_.second,tok_):false;
}