Method add

docs/app/js/sanddance-app.js:69323–69326

(...vectors)

        return this.check();
    }
    add(...vectors) {
        for (const vector of vectors)for(let i = 0; i < this.ELEMENTS; ++i)this[i] += vector[i];
        return this.check();
    }
    subtract(...vectors) {
        for (const vector of vectors)for(let i = 0; i < this.ELEMENTS; ++i)this[i] -= vector[i];
        return this.check();