64         auto high = this->lower_bound(x);  in findY()  local
66         if (high == this->end()) {  in findY()
70         if (high == this->begin()) {  in findY()
71             return high->second;  in findY()
74         auto low = high;  in findY()
80             return high->first == x ? high->second : low->second;  in findY()
82             return ((high->first - x) * low->second + (x - low->first) * high->second)  in findY()
83                     / (high->first - low->first);  in findY()
89             const S interval =  high->first - low->first;  in findY()
106             auto high2 = high;  in findY()
127                 sec = (high->second - low->second) / interval;  in findY()
132                         ? (high2->second - high->second) / (high2->first - high->first)  in findY()
141                         ? (high->second - low2->second) / (high->first - low2->first)  in findY()
173                 const T dy = high->second - low->second;  in findY()
187                 return h00 * low->second + (h10 * m0 + h11 * m1) * interval + h01 * high->second;  in findY()