Generally arithmetic operators and distinct values need to be typed by hand while variables can be chosen from the menu opening by hitting the blue rectangle on the ‘Expression’ line. The benefit is: Only existing variables are used and the curly braces plus the ‘lv=’ or ‘v=’ in case of a global variable is typed automatically.

Some examples:

To add local variables A and B and store the result in C open ‘? Set Variable’ (C) there select 'Expression' and select via the blue rectangle first 'A' then type '+' and next select 'B'. Finally the line reads {lv=A}+{lv=B}.