Invoice with "nested" taxation.

[Fred Bone]

On 11 February 2017 at 17:53, Fabio Coatti said:

> Hi all,
> I'm starting, after a couple of years, to use again gnucash; however I
> have still the same issue. I tried to find a solution (see this thread:
> http://gnucash. 1415818.n4.nabble.com/Invoice-suggestion-tt4677059.html )
> but I'm still missing a bit to have it working. Description of the
> situation: regulations require that the invoice is composed in this way:
> Description A - Amount A Description B - Amount B
> 
> Then, when describing the taxes, the printout should be something like
> this: TaxOne: C = 4%A+4%B (so far so good, no problems) TaxTwo: D =
> 22%A+22%B+22%C (ouch! it needs the value C) TaxThree: E = -(20%A+20%B)
> (ok, no problem also here)
> 
> e.g.:
> Apples € 10
> Wine € 20
> =======
> Sum: € 30
> TaxOne(C): 1,20
> SumOne: 31,20
> TaxTwo(D): 6,86
> SumTwo: 38,06
> TaxThree(E): -6
> TotalAmount: 32,06 
> 
> IIRC the problem is that taxTwo, that needs to be recorded in a specific
> account, is computed also using the value of taxOne (C). So I found no way
> to address this, also looking at the scm templates, I think because of
> this loophole. (or maybe I simply not understood the hints in above
> thread)
> 
> Is there a way to accomplish the computation and print the layout shown
> above? If not, I can try to modify the code itself, in this case a
> suggestion on how to start would be great.

What are the rounding rules? Why does the obvious way to compute TaxTwo 
directly from A and B not work?