code: func (a allocator) String() string {
  	b := &bytes.Buffer{}
  	fmt.Fprintf(b, "allocator[%d..%d]", a.low, a.high)
  
  	for low := a.low; low <= a.high; low++ {
  		high := low
  		for a.reserved(high) && high <= a.high {
  			high++
  		}
  
  		if high > low+1 {
  			fmt.Fprintf(b, " %d..%d", low, high-1)
  		} else if high > low {
  			fmt.Fprintf(b, " %d", high-1)
  		}
  
  		low = high
  	}
  	return b.String()
  }
docs: // String returns a string describing the contents of the allocator like
  // "allocator[low..high] reserved..until"
  //
  // O(N) where N is high-low