When having immutable ocl expression you always return a value - and the the OCLOperators_TrueFalse is good, but when using the MDriven Action-Language that allows updates of objects another construct is introduced to compact things even more:

boolean.case(dothis):boolean

Example:

let x=SomeObject.SomeEnum in (  
  (x=#Enum1).case(SomeObject.DoYourThing1);  
  (x=#Enum2).case(SomeObject.DoYourThing2);  
  (x=#Enum3).case(SomeObject.DoYourThing3) 
)