key(key) {
		if (this.children === undefined) this.children = new Map();
		const child = this.children.get(key);
		if (child !== undefined) return child;
		const newChild = new ObjectStructure();
		this.children.set(key, newChild);
		return newChild;
	}
@param {keyof T} key key
@returns {ObjectStructure<T>} object structure