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Semantics for ECMAScript 5.0

Install and Usage
See `INSTALL` for installation instruction.

s5 options are implemented as stack manipulation so the order of
options matters. Here are a few examples (in `src/` directory, and a
js file `test.js` exists):

Desugar a Javascript program and print the S5 source.

    $ ./s5.d.byte -desugar test.js -print-src

Desugar a Javascript program, wrap with an Environment and do evaluation.

    $ ./s5.d.byte -desugar test.js -env ../envs/es5.env -apply -eval

Desugar a Javascript program, wrap with an Environment, apply
optimizations and finally print out the optimized code (excluding the

    $ ./s5.d.byte -desugar test.js -env ../envs/es5-optimization.env -apply -opt-restore-id -opt-restore-function -opt-propagate-const -opt-clean-deadcode -print-user-s5

Slimming Language Options

All optimization options start with "-opt-". There are a few options
not compatible with the optimization flags:

1. `-internal-env env-vars`. This option introduces identifiers of
es5.env in the nested environment for `eval`. It has a huge negative
impact on the constant propagation.
2. `-env es5.env`. There are several `env` files in env/
directory. Optimization flags only work on `es5-optimization.env`,
which is similar to `es5.env`. The only difference is that it excludes
built-in `eval` support (-opt-restore-id would not work if `es5.env`
is used). See previous usage examples for how to pass a correct