diff -r 07c16ec15a25 -r 58ec6da75f6f task4/solution13/src/org/apidesign/apifest08/currency/IExchangeRateEngine.java
--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/task4/solution13/src/org/apidesign/apifest08/currency/IExchangeRateEngine.java	Sat Oct 11 23:38:46 2008 +0200
@@ -0,0 +1,18 @@
+package org.apidesign.apifest08.currency;
+
+/**
+ * Interface for exchange rate engine.
+ * 
+ * @author arnostvalicek
+ */
+public interface IExchangeRateEngine {
+    
+    /**
+     * Get exchange rate for conversion from <code>fromCurrency</code> to <code>toCurrency</code>.
+     * 
+     * @param fromCurrency From currency.
+     * @param toCurrency To currency.
+     * @return Returns <code>ExchangeRate</code> if exchange rate is known or <code>null</code> if exchanger rate is not known.
+     */
+    public ExchangeRate getExchangeRate(ConvertorCurrency fromCurrency, ConvertorCurrency toCurrency);
+}