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You can change a keymap by editing code of keymap.c like following.
This is a keymap for AEK, however, also used for other keyboards.
-How to define the keymap is probably obvious. You can find key symbols in usb_keycodes.h.
+How to define the keymap is probably obvious. You can find key symbols in keycode.h.
If you want to define some keymaps than just one, see hhkb/keymap.c and
macway/keymap.c as examples. Keymap(layer) switching may needs a bit of
effort at this time.