"Not" is the usual Not operator found in most languages.

However, there are a few things to point out when writing expressions using True, False, nullable booleans, and Not.

Avoid the Risk With Nullable Booleans

Consider xxxx.DisableEditing = false.

When you think you want to use "a boolean" with = true/false, don't—it’s almost never correct.

The reason is that a nullable boolean is tri-state: False, True, and Null. If an expression is xxxx.DisableEditing, it is false for BOTH False and Null.

So, almost always use "xxxx.DisableEditing" or "not xxxx.DisableEditing" (there are exceptions, but not many).

For example:

self.ValidTo.notNull and
self.AddedTo.isNull and
(self.Agreement.Facilitator.DisableEditing = false)

If DisableEditing is Null, then "(self.Agreement.Facilitator.DisableEditing = false)" is actually True (the above false/null problem):

self.ValidTo.notNull and
self.AddedToJournalSearch.isNull and
not self.Agreement.Facilitator.DisableEditing