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The overlay approach to dynamic spectrum access recently proposed in information theory allows both primary users (PUs) and secondary users (SUs) to simultaneously access the same spectrum with comparable power levels. However, the information theory approach is based on idealized assumptions that are difficult to be satisfied in practice. We propose a practical scheme with the overlay flavor for multi-hop wireless networks subject to two challenging design constraints: (1) PUs can not be modified, and (2) the performance of PUs can not degrade. Using the notion of credit in our scheme, SUs first help with the traffic delivery for PUs, and in return SUs later are allowed to access the spectrum in a manner disruptive to PUs. Specifically, SUs earn credit for helping PUs and consume credit for disrupting PUs. Moreover, we propose a distributed credit diffusion algorithm for SUs to share credit, and we modify a widely used reactive routing protocol to satisfy the design constraints. This overlay scheme is then integrated with the widely used interweave scheme to form the Credit-based Overlay and Interweave (CONI) protocol. Simulation results show that CONI can significantly benefit both PUs and SUs.