Smallish unintended effect: I wanted to have an action set a bool column to undefined. I couldn't.
Code:
calibre, version 5.6.0
ERROR: Undefined value: Predefined options chosen without specifying a value
Search and replace worked, but it felt kind of like using a sledgehammer to put up a thumbtack.
Perhaps could be also added a fourth option in Single Field Edit: "Clear current value"?