[simple-book-add-txn] keep trying until txn balanced

because using weird fractions for amount eg amt=222/13 will lead to
imbalanced 0.01 transactions

doc/examples/simple-book-add-txn.scm

@@ -99,31 +99,37 @@
   (action-fn acc-EXP-LEAF)
   (action-fn acc-EXP-TAX))
 
-(define txn (xaccMallocTransaction book))
-
 (define description (get-line "Description"))
-(define net-amount (get-amount "Amount, without tax"))
-(define tax-amount (* net-amount 1/10))
-(define total-amount (+ tax-amount net-amount))
-
-(accounts-action xaccAccountBeginEdit)
-(xaccTransBeginEdit txn)
-(xaccTransSetCurrency txn (xaccAccountGetCommodity acc-BANK))
-(xaccTransSetDatePostedSecsNormalized txn (current-time))
-(xaccTransSetDescription txn description)
-(add-to-transaction book txn acc-BANK (- total-amount) "from bank")
-(add-to-transaction book txn acc-EXP-LEAF net-amount "expense net")
-(add-to-transaction book txn acc-EXP-TAX tax-amount "tax paid")
-(newline)
-(gnc:dump-transaction txn)
-
-(cond
- ((get-binary-response "Please confirm transaction [YN]")
-  (xaccTransCommitEdit txn))
- (else
-  (xaccTransRollbackEdit txn)
-  (xaccTransDestroy txn)))
-(accounts-action xaccAccountCommitEdit)
+
+(let lp ()
+  (define txn (xaccMallocTransaction book))
+  (define net-amount (get-amount "Amount, without tax"))
+  (define tax-amount (* net-amount 1/10))
+  (define total-amount (+ tax-amount net-amount))
+
+  (xaccTransBeginEdit txn)
+  (xaccTransSetCurrency txn (xaccAccountGetCommodity acc-BANK))
+  (xaccTransSetDatePostedSecsNormalized txn (current-time))
+  (xaccTransSetDescription txn description)
+  (add-to-transaction book txn acc-BANK (- total-amount) "from bank")
+  (add-to-transaction book txn acc-EXP-LEAF net-amount "expense net")
+  (add-to-transaction book txn acc-EXP-TAX tax-amount "tax paid")
+  (newline)
+  (gnc:dump-transaction txn)
+
+  (cond
+   ((not (xaccTransIsBalanced txn))
+    (display "WARNING: transaction is not balanced. Try again.\n")
+    (xaccTransRollbackEdit txn)
+    (xaccTransDestroy txn)
+    (lp))
+   ((get-binary-response "Please confirm transaction [YN]")
+    (accounts-action xaccAccountBeginEdit)
+    (xaccTransCommitEdit txn)
+    (accounts-action xaccAccountCommitEdit))
+   (else
+    (xaccTransRollbackEdit txn)
+    (xaccTransDestroy txn))))
 
 ;; (gnc:dump-book)
 (when (qof-book-session-not-saved book)