  We first prove the Grinberg-Kazhdan formal arc theorem without any
assumptions on the characteristic. This part of the article is equivalent to
arXiv:math-AG/0203263.
  Then we try to clarify the geometric ideas behind the proof by introducing
the notion of Newton groupoid (which is related to Newton's method for finding
roots). Newton groupoids are certain groupoids in the category of schemes
associated to any generically etale morphism from a locally complete
intersection to a smooth variety.
