Add Example to the Rule specification Docstring of the Variable class

[popescu-v]

Description

Currently, there is no rule specification example in the docstring of the Variable.rule attribute. The goal of this issue is to add such an example in order to separate the "classic" rules from the reference rules.

Questions/Ideas

• Such an example could be something like: Sum(Var1, Var2) for a standard rule, or [Identifier] for a reference rule added to the docstring of Variable.rule.