calculate "around" a variable

MFelix
Good point. I guess it comes down to nomenclature, to what exactly your definition of a constant entails.
What I said, or meant to say, is that once the variable is assigned a value (at creation) its contents are immutable. I am referring here to each execution instance of a variable. An instance of that variable exists between the point where it is created/assigned a value (which happens simultaneously) and the point where it is "used up" through RETURN. 

When in a calculated column we have something like:

VAR CurrentID = 'Table'[ID]
RETURN
    CALCULATE (
        SUM ( 'Table'[Sales] );
        FILTER ( 'Table'; 'Table'[ID] < CurrentID )
    )

we'll have, for each row, a different instance of the variable CurrentID. I am limiting my definition of constant to what happens within a row, which is the effective existence of each instance of CurrentID. And I emphasized that aspect because it was the culprit
in the issue presented by dedelman_clng   
 
You are extending the definition to encompass all execution instances. If you consider it so, it is true that each instance will (possibly) be created with a different value. But that value will be immutable during a particular instance's life.