Dixon's Identity/Gaussian Binomial Form/Formulation 1

Theorem
For $l, m, n \in \Z_{\ge 0}$:


 * $\ds \sum_{k \mathop \in \Z} \paren {-1}^k \dbinom {m - r - s} k_q \dbinom {n + r - s} {n - k}_q \dbinom {r + k} {m + n}_q = \dbinom r m_q \dbinom s n_q$

where $\dbinom r m_q$ denotes a Gaussian binomial coefficient