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diff --git a/src/exchangedb/exchange_do_amount_specific.sql b/src/exchangedb/exchange_do_amount_specific.sql
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+++ b/src/exchangedb/exchange_do_amount_specific.sql
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+--
+-- This file is part of TALER
+-- Copyright (C) 2014--2022 Taler Systems SA
+--
+-- TALER is free software; you can redistribute it and/or modify it under the
+-- terms of the GNU General Public License as published by the Free Software
+-- Foundation; either version 3, or (at your option) any later version.
+--
+-- TALER is distributed in the hope that it will be useful, but WITHOUT ANY
+-- WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS FOR
+-- A PARTICULAR PURPOSE.  See the GNU General Public License for more details.
+--
+-- You should have received a copy of the GNU General Public License along with
+-- TALER; see the file COPYING.  If not, see <http://www.gnu.org/licenses/>
+--
+
+--------------------------------------------------------------
+-- Taler amounts and helper functions
+-------------------------------------------------------------
+
+CREATE OR REPLACE FUNCTION amount_normalize(
+    IN amount taler_amount
+  ,OUT normalized taler_amount
+)
+LANGUAGE plpgsql
+AS $$
+BEGIN
+  normalized.val = amount.val + amount.frac / 100000000;
+  normalized.frac = amount.frac % 100000000;
+END $$;
+
+COMMENT ON FUNCTION amount_normalize
+  IS 'Returns the normalized amount by adding to the .val the value of (.frac / 100000000) and removing the modulus 100000000 from .frac.';
+
+CREATE OR REPLACE FUNCTION amount_add(
+   IN a taler_amount
+  ,IN b taler_amount
+  ,OUT sum taler_amount
+)
+LANGUAGE plpgsql
+AS $$
+BEGIN
+  sum = (a.val + b.val, a.frac + b.frac);
+  CALL amount_normalize(sum ,sum);
+
+  IF (sum.val > (1<<52))
+  THEN
+    RAISE EXCEPTION 'addition overflow';
+  END IF;
+END $$;
+
+COMMENT ON FUNCTION amount_add
+  IS 'Returns the normalized sum of two amounts. It raises an exception when the resulting .val is larger than 2^52';
+
+CREATE OR REPLACE FUNCTION amount_left_minus_right(
+  IN  l taler_amount
+ ,IN  r taler_amount
+ ,OUT diff taler_amount
+ ,OUT ok BOOLEAN
+)
+LANGUAGE plpgsql
+AS $$
+BEGIN
+
+IF (l.val > r.val)
+THEN
+  ok = TRUE;
+  IF (l.frac >= r.frac)
+  THEN
+    diff.val  = l.val  - r.val;
+    diff.frac = l.frac - r.frac;
+  ELSE
+    diff.val  = l.val  - r.val - 1;
+    diff.frac = l.frac + 100000000 - r.frac;
+  END IF;
+ELSE
+  IF (l.val = r.val) AND (l.frac >= r.frac)
+  THEN
+    diff.val  = 0;
+    diff.frac = l.frac - r.frac;
+    ok = TRUE;
+  ELSE
+    diff = (-1, -1);
+    ok = FALSE;
+  END IF;
+END IF;
+
+RETURN;
+END $$;
+
+COMMENT ON FUNCTION amount_left_minus_right
+  IS 'Subtracts the right amount from the left and returns the difference and TRUE, if the left amount is larger than the right, or an invalid amount and FALSE otherwise.';