code: func (s *session) setCompPtr(level int, ik internalKey) {
  	if level >= len(s.stCompPtrs) {
  		newCompPtrs := make([]internalKey, level+1)
  		copy(newCompPtrs, s.stCompPtrs)
  		s.stCompPtrs = newCompPtrs
  	}
  	s.stCompPtrs[level] = append(internalKey{}, ik...)
  }
docs: // Set compaction ptr at given level; need external synchronization.