Referencing property of a concept as an Expression

My concept:

concept Attribute extends Expression implements INamedConcept


properties:
string name;
int average;

children:
Attribute dependsOn;

I have an instance of it, named "Age".

Another concept I have is "Strategy". The purpose of it is to define how I calculate some rating based on attributes.

concept Strategy extends Expression implements INamedConcept

children:
Expression rating;

In my instance of a Strategy I'd like to be able to define rating like this:

rating = Age.average - Age + dependsOn.average

Referencing Age itself is no problem. But referencing properties of a concept itself is another matter, and I don't see how to do that.

Any suggestions?