Well, there are few restrictions, currently:
∘ It is a single derived function applied to one or two arguments.
∘ The arguments can be any of `⍺` `⍶` `⍵` `⍹`, with underlines for magic arrays, or constant values. At least one argument must be underlined.
∘ No argument token can be used twice, including with an underline.
∘ `⍵` must be used. If `⍵⍵` is used, `⍺⍺` must be as well.