code: func (bitmap *bitmap) set(index int, bit int) {
  	if index >= bitmap.Size {
  		panic("index out of range")
  	}
  
  	div, mod := index>>3, index&0x07
  	shift := byte(1 << uint(7-mod))
  
  	bitmap.data[div] &= ^shift
  	if bit > 0 {
  		bitmap.data[div] |= shift
  	}
  }
docs: // set sets the bit at index `index`. If bit is true, set 1, otherwise set 0.