See the racket docs:
https://pkgd.racket-lang.org/pkgn/package/circuit-playground
If you make big changes to the api, you'll need to rebuild the firmware. Not a pretty process, but I'll document anyway for my own benefit:
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Make sure the compiler outputs tslib.py to the CPX (edit circuit-playground.rkt)
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Make sure the arm gcc is on your path
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Run ./scripts/redo-firmware.rb
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Put the CPX into bootloader mode, put ./tools/firmware.uf2 onto the CPX
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If all is good, commit the new firmware to github