public PartialTrustEvaluationStrategy(string expression, VsaEngine engine)
: base(engine)
{
//
// Following is equivalent to declaring a "var" in JScript
// at the level of the Global object.
//
_scope = (GlobalScope)engine.GetGlobalScope().GetObject();
_oldContextField = _scope.AddField("$context");
_newContextField = _scope.AddField("$");
_expression = expression;
}