IMO it should be like Fennel and just support `lambda` and `λ`. The latter is not even that hard to type, in virtually any free (libre) OS you can just
$ echo '
<Multi_key> <Y> <Y> : "Λ" # GREEK CAPITAL LETTER LAMDA
<Multi_key> <y> <y> : "λ" # GREEK CAPITAL LETTER LAMDA
' >> ~/.XCompose
If you’re limited by a nonfree OS you should be able to patch the problem with free duct tape like Karabiner or AutoHotKey.
And in vim you can just
in insert mode (which can be done with option-l on macOS if you `inoremap ¬ <C-K>*l` https://apple.stackexchange.com/questions/230422/type-lambda...).