code: private void copyKeys(int upToKeyPosition)
      {
          if (copyFromKeyPosition >= upToKeyPosition)
              return;
  
          int len = upToKeyPosition - copyFromKeyPosition;
          assert len <= FAN_FACTOR : upToKeyPosition + "," + copyFromKeyPosition;
  
          ensureRoom(buildKeyPosition + len);
          if (len > 0)
          {
              System.arraycopy(copyFrom, copyFromKeyPosition, buildKeys, buildKeyPosition, len);
              copyFromKeyPosition = upToKeyPosition;
              buildKeyPosition += len;
          }
      }
docs: copy keys from copyf to the builder, up to the provided index in copyf (exclusive)