Paul Sally explains this in these notes:

http://link.springer.com/article/10.1023%2FA%3A1007583108067#page-1

I am afraid he excludes residue characteristic two:(

Paul Sally explains this in these notes:

http://link.springer.com/article/10.1023%2FA%3A1007583108067#page-1

I am afraid he excludes residue characteristic two there:(

But this seems to be more general: http://www.jstor.org/stable/2373536?seq=1