The present invention provides a system, computer program product and a computer implemented method for prioritizing code fragments based on the use of a software oracle and on a correlation between the executed code fragments and the output they produce. Also described is a computer-implemented method generates additional user inputs based on execution information associated with path constraints and based on information from the oracle. Advantageously, the embodiment is useful in a test generation tool that generated many similar inputs when a failure-inducing input is found, in order to enhance fault localization. Further, described is a computer-implemented flow for extending the existing idea of concolic testing to applications that interact with persistent state.