code: func (m *symbol) numEmptyModules() int {
  	var count int
  	for y := 0; y < m.symbolSize; y++ {
  		for x := 0; x < m.symbolSize; x++ {
  			if !m.isUsed[y+m.quietZoneSize][x+m.quietZoneSize] {
  				count++
  			}
  		}
  	}
  
  	return count
  }
docs: // numEmptyModules returns the number of empty modules.
  //
  // Initially numEmptyModules is symbolSize * symbolSize. After every module has
  // been set (to either true or false), the number of empty modules is zero.