Nothing but relativity, redux
A previous paper demonstrated that spacetime transformations consistent with the principle of relativity could be derived without assuming explicitly the constancy of the speed of light. Here, we correct an error in the earlier paper while showing that this derivation can be done under weaker assumptions, in particular, without an implicit assumption of differentiability or even continuity for the spacetime mapping. Hence, these historic results could have been derived centuries ago, even before the advent of calculus.