Is the composition of chain boundaries a boundary?

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Suppose I have two singular chains from the $n$-simplex into itself, $$\sigma, \tau: \Delta^n \to \Delta^n$$ and that I know that these are boundaries, $\sigma = \partial s$ and $\tau = \partial t$ for some $s, t: \Delta^{n+1} \to \Delta^n.$ What can I say about the composition $\sigma \circ \tau$? Is that a boundary?