Executability provides an important mechanism for validating formal specifications and allows such specifications to serve as prototypes and test oracles. In this case study, we used the jmle tool to execute the JML specification of an electronic purse application written in the Java Card dialect of Java. This effort resulted in numerous improvements to the specification and to the jmle tool itself, as well as insight into how executability can contribute to the use of formal methods in the software development process.