CalculateableValue — The type of attribute that contains an
expression to be calculated at run time.
The type of a string constant or an expression. If the value of
this type is started with '--', the rest of string is the text of
an expression to be calculated in order to get the effective value
of the property. If the value of this type is not started with
'--', the effective value is the original string itself.
The type identifier 'CalculateableValue' is introduced only for
diagnostic purposes, you will never use it in VSPX code. When
Virtuoso server tries to compile an invalid VSPX page, you might
see a diagnostic messages like 'the value of attribute X of a
control Y does not match pattern ... for type CalculateableValue'.
If you see this then you should check the syntax of the value of
the specified attribute.