`lowerIncludes`を `caseIncludes`に変更。 ```ts /** * This function uses case-sensitive logic if a second argument has an upper case. Otherwise, uses case-insensitive logic. */ const caseIncludes = (one: string, other: string): boolean => { const lowerOther = other.toLowerCase(); return lowerOther === other ? one.toLowerCase().includes(lowerOther) : one.includes(other); }; ```