Syntax
not(<boolean expression>)
set aBoolean = anotherBoolean.not()
SemanticsNegates the <boolean expression> or a boolean value.
SubstitutablesanotherBoolean Can be any variable or object attribute having the type Boolean. 
Examples

Returns false:

set inverted = not(true)

Assigns the negation of the boolean value of x to y:

set y = x.not();

Example File (Builder project E2E Action Language/BaseTypes):

<your example path>\E2E Action Language\BaseTypes\uml\boolean.xml