(q;q)a1+a2+⋯+an(q;q)a1⁢(q;q)a2⁢⋯⁢(q;q)an= coeff. of ⁢x10⁢x20⁢⋯⁢xn0⁢ in ⁢∏1≤j<k≤n(xjxk;q)aj⁢(q⁢xkxj;q)ak.