23.4 Zero-Length Extents

Extents can be zero-length, and will end up that way if their endpoints are explicitly set that way or if their detachable property is nil and all the text in the extent is deleted. (The exception is open-open zero-length extents, which are barred from existing because there is no sensible way to define their properties. Deletion of the text in an open-open extent causes it to be converted into a closed-open extent.) Zero-length extents are primarily used to represent annotations, and behave as follows:

1. Insertion at the position of a zero-length extent expands the extent if both endpoints are closed; goes after the extent if it is closed-open; and goes before the extent if it is open-closed.
2. Deletion of a character on a side of a zero-length extent whose corresponding endpoint is closed causes the extent to be detached if it is detachable; if the extent is not detachable or the corresponding endpoint is open, the extent remains in the buffer, moving as necessary.

Note that closed-open, non-detachable zero-length extents behave exactly like markers and that open-closed, non-detachable zero-length extents behave like the “point-type” marker in Mule.