Method decreaseUsedCount

BambooTracker/module/pattern.hpp:42–42

	inline int increaseUsedCount() noexcept { return ++usedCnt_; }
	inline int decreaseUsedCount() noexcept { return --usedCnt_; }
	inline int getUsedCount() const noexcept { return usedCnt_; }

	Step& getStep(int n);