Properties to be checked among variables in the predicate. Properties should be moved to a XML on a format on their
own in the future. However, lot of changes are foreseen here in the immediate future so it's easy to keep them in the
current format. Properties are a strings in the openschema xml definition file (OpenSchema.xsd). The class
Property is abstract and contains a parse static method that will translate the string into a tree
of Property subclasses, defined with package access rights only.