modal

Using the same type of distributional criterion, we could argue that only a Verb (in its base form) can occur in the position marked — in (23) below to complete the sentence:

(23) They/it can —

…

Conversely, the only type of word which could be used to begin a three-word sentence such as (25) below:

(25) — I be frank?

is a Modal: cf. …

Modal screen elements are subtrees which, when activated, disable all elements external to them. Examples of modals are yes-no message boxes and the application itself.