[Feature Request] Remove 'seal' from XXXVariable to enable inheritance and add expression

[abcjjy]

Expression SO can inherit variables and make SOA more powerful and flexiable.

For example

class ComparisonExpression : BoolVariable
{
  public BaseVariable var1;
  public CompareOperator op;
  public BaseVariable var2;

  public override bool Value
  {
    get {
    // compare var1 and var2
    }
  }
}

[lgarczyn]

I agree. This platform is interesting for its hackability, I don't see the point in sealed classes here.