code: func (b *Buffer) Alloc(n int) []byte {
  	if n < 0 {
  		panic("leveldb/util.Buffer.Alloc: negative count")
  	}
  	m := b.grow(n)
  	return b.buf[m:]
  }
docs: // Alloc allocs n bytes of slice from the buffer, growing the buffer as
  // needed. If n is negative, Alloc will panic.
  // If the buffer can't grow it will panic with bytes.ErrTooLarge.