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Design
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The module does not reimplement tax formulas. Invoice line helper amounts are
obtained from ``account.invoice.line._get_taxes`` and classified with the tax
group codes supplied by ``account_do``. This preserves Tryton's handling of
fixed, included, compound, dated and rounded taxes.
All informational tax fields and the line total share one multi-field getter,
so the core tax result is classified only once per invoice line read.

The displayed due date is the last maturity date. For posted invoices it is
read from the payable move lines. For draft invoices it is computed by the
complete payment term using the invoice amount and currency.