  We introduce a first-order theory $\mathsf{Seq}$ which is mutually
interpretable with Robinson's $\mathsf{Q}$. The universe of a standard model
for $\mathsf{Seq}$ consists of sequences. We prove that $\mathsf{Seq}$ directly
interprets the adjuctive set theory $\mathsf{AST}$, and we prove that
$\mathsf{Seq}$ interprets the tree theory $\mathsf{T}$ and the set theory
$\mathsf{AST + EXT}$.
