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Developer builds of Chrome now support a simple remote debugging protocol . Already this protocol supports - JSON RPC over WebSockets. This protocol provides much of what is needed to build high quality debugging tools for ClojureScript. Coupled with source-mapping and analysis, it should be possible to develop sexpr level stepping and debugging of ClojureScript source.
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http://code.google.com/chrome/devtools/docs/protocol/0.1/index.html