code: public RunT getNearestOldBuild(int n) {
          SortedMap<Integer, ? extends RunT> m = _getRuns().tailMap(n);
          if (m.isEmpty())
              return null;
          return m.get(m.firstKey());
      }
docs: Gets the latest build #m that satisfies {@code m&lt;=n}.
  
  This is useful when you'd like to fetch a build but the exact build might
  be already gone (deleted, rotated, etc.)
  @see LazyBuildMixIn#getNearestOldBuild