Code review request of procedural macro

I just published a new crate here (github). In summary, it is a crate to facilitate a new way of handling errors in Rust, but before I start advertising it as such I'd like to get feedback from someone who is more experienced with procedural macros than I am (this is my first such macro). Detailed information about what the crate is is available on Github and (and once it gets around to generating documentation for my cratee).