Is PUT atomic?

This is probably better asked to a different group than this one, but I
hope someone knows the answer anyway: 

Does HTTP say anything at all, one way or the other, about whether a PUT is
atomic?  That is to ask, suppose two parties, A and B, each do a PUT on the
same resource.  Does HTTP say whether afterwards, if party C does a GET,
the result will either be what A sent, or B, but not some horrible
conflation of both?