The Expression Editor appears when you need to enter a value for an action or a condition. You will therefore get it when you are using the event editor.

The expression editor works like a big calculator. The current expression is displayed in the white edit zone and you can click any of the buttons to enter a new value.
The number buttons are used to enter numbers, and the function buttons to enter functions, like Sine and Cosine. Some functions ask for a parameter, the parameter is delimited by < and > and is automatically selected when you choose the function. Replace the content of < and > by the parameter. To select the next < and > click on the Next button.
Some actions or conditions ask for text expressions : you then use strings enclosed in quotes like “This string”, and the functions ending by $.

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Retrieve data from object
This button is very important : it opens a dialog box presenting you with all the objects of the application. Right-click on one of the objects. This opens a popup menu presenting the possible expressions of the object : you just have to choose what value you need to retrieve from the object. Click on OK, and a new function appears in the edit zone.

Note:
If “Valid expression” appears then your expression is correctly written. If “Syntax error” appears then you have made a mistake somewhere in your expression : carefully check every element of the expression to find it.