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Overlays are virtual, re-configurable architectures that overlay on top of physical FPGA fabrics. An overlay that is specialized for an application, or a class of applications, offers both fast reconfiguration and minimized performance penalty. Such an overlay is usually implemented by hardware designers in hardware "assembly" languages at register-transfer level (RTL). This short article proposes an idea for a software programmer, instead of hardware designers, to quickly implement an application-specific overlay using high-level customizable IPs. These IPs are expressed succinctly by a specification language, whose abstraction level is much higher than RTL but can nonetheless expresses many performance-critical loop and data optimizations on FPGAs, and thus would offer competitively high performance at a much lower cost of maintenance and much easier customizations. We propose new language features to easily put the IPs together into an overlay. A compiler automatically implements the specified optimizations to generate an efficient overlay, exposes a multi-tasking programming interface for the overlay, and inserts a runtime scheduler for scheduling tasks to run on the IPs of the overlay, respecting the dependences between the tasks. While an application written in any language can take advantage of the overlay through the programming interface, we show a particular usage scenario, where the application itself is also succinctly specified in the same language. We describe the new language features for expressing overlays, and illustrate the features with an LU decomposer and a convolutional neural network. A system is under construction to implement the language features and workloads.