By selecting it from a list of variables read by VU when it opens a file. This list is available by pressing mouse button 3 whenever the cursor is over a field where an expression is needed. Some currently used formulas are inserted to this list when a file is read. Each new expression entered by the user is then added to the list. By using the same syntax VU uses.
Every variable is numbered according to the order in which it was read.
This way, one can read a different solution file and still use the same expression. There is no need to change the variable names in every single expression.