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20130902 (Monday, 02 September 2013)
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Whether unit prices of sales items include VAT or not
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The `item_vat` checkbox on a VatDocument (e.g. an order or an 
invoice) decides whether unit prices of sales items include VAT 
or not.

With Lino you can decide this per document.
In fact the end-user doesn't want to decide, they just want it to 
have the right default value.

Until now the default value was *hard-coded* and *always No*.

But sometimes it is Yes.

And this decision should be configurable by local site managers, 
i.e. not by modifying application code, 
only by modifying their :xfile:`settings.py`.

Solution so far: 
I added a new Site configuration method 
:meth:`lino.modlib.vat.SiteMixin.get_item_vat` which is documented 
as follows:

    Expected to return the default value for the `item_vat` field.
    Called on every new voucher whose model is subclass of 
    `lino.modlib.vat.models.VatDocument`.

But that's maybe not a final solution. I guess that there will be 
system admins who want to decide this when the customer or at 
the vat_regime has been selected...

Another question is: how to configure whether the prices in 
the products catalog are maintained VAT included or not. 
And if they are, then we need a default VAT rate,
and remove the VAT when selling to a vat-exempt customer.
And be careful when the default VAT rate changes.
And must we store this information per product instance?

Meditate... continued :blogref:`tomorrow <20130903>`.