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After in Python 3.2 the accumulate() function has been added to itertools module, for self-evident reasons of completeness and symmetry we could add in 3.3 the inverse decumulate() function, which, given the iterable argument p, yields p[0], p[1] - p[0], p[2] - p[1]... Its equivalent python form is in the attached file decumulate.py, below.

OK, looking at the code I realised what you're trying to get at is the idea of reporting the differences between values in a series, such that:
x = list(accumulate(seq))
assert x == list(accumulate(differences(x)))
I don't think the use cases are there to justify the addition (not every iterator we can think of should be a building block in itertools), but if such a thing were to be added, "differences" would convey the intention significantly better than the invented term "decumulate".

I don't think this should go in itertools. It could go in the recipes at the bottom of the itertools doc, if you can provide some valid use cases (and possibly a patch too).

I also agree that the itertools module doesn't need this. (Reasons of "symmetry" are insufficient to make the module fatter that it already is). In addition, there are already a number of simple ways to do this.