Method find_adj

src/include/interval_set.h:314–323

  }
  
  auto find_adj(T start) const {
    auto p = m.lower_bound(start);
    if (p != m.begin() &&
        (p == m.end() || p->first > start)) {
      --p;   // might touch?
      if (p->first + p->second < start)
        ++p; // it doesn't.
    }
    return p;
  }
  
  auto find_adj_m(T start) {
    auto p = m.lower_bound(start);