func (f *Field) flushableLine(j int) bool {
	for i := 0; i < fieldBlockNumX; i++ {
		if f.blocks[i][j] == BlockTypeNone {
			return false
		}
	}
	return true
}
func (f *Field) flushableLine(j int) bool {
	for i := 0; i < fieldBlockNumX; i++ {
		if f.blocks[i][j] == BlockTypeNone {
			return false
		}
	}
	return true
}
// flushableLine returns a boolean value indicating whether
// the line j is flushabled or not.