yes, the properties can be initialized with expressions also including constants as long as you assign the expression directly to the property (e.g. to Bounds).
In turn, the values subordinated to the property (e.g. x2) don't accept expressions other than literals (e.g. numbers). These subordinated values are exposed for the convenience only and they are mapped automatically to a single literal which is then assigned to the original property.
Here you assign an expression to the property Bounds to calculate its complete rect value. Please note the usage of the rect() instant constructor to create the rect value. Also please note, when you assign an expression other than literal to the property, the subordinated values (e.g. x2) disappear later.
Does it help you further?