void backward_fill(Iterator it, int row, int first, int last){
value_type acc = m_traits.default_value();
--it;
for(int i = first - 1; i >= last; --i, --it){
acc = m_traits(*it, acc);
m_table[row][i] = acc;
}
}
void forward_fill(Iterator it, int row, int first, int last){
value_type acc = m_traits.default_value();
m_table[row][first] = acc;
for(int i = first + 1; i < last; ++i, ++it){
acc = m_traits(acc, *it);
m_table[row][i] = acc;
}
}