“…Given an appropriate (case-specific) replacement for the notation of "affine mapping," one can formulate notions of "differentiation" in many settings that do not necessarily involve linear spaces; examples of such "qualitative" metric differentiation results include [71,46,16,73,45,51,18,18,19,20]. Corresponding results about quantitative differentiation, which lead to refined (often quite subtle and important) rigidity results can be found in [7,43,67,59,21,52,74,75,22,53,17,29,62,30,31,24,23,2,54,32]. Due to the prominence of this topic and the fact that many of the quoted results are probably not sharp, it would be of interest to develop new methods to prove sharper quantitative differentiation results.…”