code: func (f *Field) flushLine(j int) bool {
  	for i := 0; i < fieldBlockNumX; i++ {
  		if f.blocks[i][j] == BlockTypeNone {
  			return false
  		}
  	}
  	for j2 := j; 1 <= j2; j2-- {
  		for i := 0; i < fieldBlockNumX; i++ {
  			f.blocks[i][j2] = f.blocks[i][j2-1]
  		}
  	}
  	for i := 0; i < fieldBlockNumX; i++ {
  		f.blocks[i][0] = BlockTypeNone
  	}
  	return true
  }
docs: // flushLine flushes the line j if possible, and if the line is flushed,
  // the other lines above the line go down.
  //
  // flushLine returns a boolean value indicating whether
  // the line is actually flushed.