  For the importance of differentiation theorems in metric spaces (starting
with Pansu Rademacher type theorem in Carnot groups) and relations with
rigidity of embeddings see the section 1.2 in Cheeger and Kleiner paper
arXiv:math/0611954 and its bibliographic references.
  Here we propose another type of differentiation theorem, which does not
involve measures. It is therefore different from Rademacher type theorems.
Instead, this differentiation theorem (and the concept of uniformly topological
derivable function) is formulated in terms of filters in topological spaces.
