docs: journal: Document exact assertions

site/doc/1.11/journal.md

@@ -452,13 +452,49 @@ The asserted balance must be a simple single-commodity amount, and in
 fact the assertion checks only this commodity's balance within the
 (possibly multi-commodity) account balance. We could call this a partial
 balance assertion. This is compatible with Ledger, and makes it possible
-to make assertions about accounts containing multiple commodities.
+to make assertions about accounts containing multiple commodities. To
+assert the balance of more than a single commodity, you can add multiple
+postings (with amount 0 if necessary).
 
-To assert each commodity's balance in such a multi-commodity account,
-you can add multiple postings (with amount 0 if necessary). But note
-that no matter how many assertions you add, you can't be sure the
-account does not contain some unexpected commodity. (We'll add support
-for this kind of total balance assertion if there's demand.)
+To instead assert a commodity's balance to the exclusion of all others
+in an account, use the exact assertion form `==EXPECTEDBALANCE` (note
+the doubled equals sign). This, unlike multiple partial assertions,
+ensures that the account contains no unexpected commodities -- or
+equivalently, that the balance of any other commodity is 0.
+
+``` {.journal}
+2013/1/1
+  a   $1
+  a    1€
+  b  $-1
+  c   -1€
+
+2013/1/2  ; These assertions succeed
+  a    0  =  $1
+  a    0  =   1€
+  b    0 == $-1
+  c    0 ==  -1€
+
+2013/1/3  ; This assertion fails as 'a' also contains 1€
+  a    0 ==  $1
+```
+
+Unfortunately, as of now, there is no way to specify that an account
+contains exactly values of multiple commodities. Until and unless that
+capability is added, the best workaround is to isolate each commodity
+into individual subaccounts:
+
+``` {.journal}
+2013/1/1
+  a:usd   $1
+  a:euro   1€
+  b
+
+2013/1/2
+  a        0 ==  0
+  a:usd    0 == $1
+  a:euro   0 ==  1€
+```
 
 #### Assertions and subaccounts