Syntax
set aBoolean = <boolean expression>;
Semantics

Evaluates <boolean expression> to true or false and assigns the boolean value to aBoolean. Boolean variables can be used in boolean expressions .

Examples
set b1 = b2 and b3; 
set b4 = b2 or b3; 
set b5 = b2 and (b3 or b4); 
set b6 = not (b2);
set b7 = 2 < 1;

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

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