Full support of variables for predicate’s reference

[Seddryck]

At the moment when defining a predicate’s reference, you can’t systematically set it to a variable. This support should be improved to handle variables in any case.

<predicate operand="#0">
   <equal>@var</equal>
</predicates>

[Seddryck]

Build is available at https://ci.appveyor.com/project/Seddryck/nbi/builds/19805551 or nuget 1.19-beta0090