public override void Evaluate(Stack state, FormulaEngine engine)
        {
            var     namedRef = (Reference)engine.ReferenceFactory.Named(_name);
            Formula target   = engine.GetFormulaAt(namedRef);

            if (target != null)
            {
                var      selfRef = (NamedReference)target.SelfReference;
                IOperand op      = selfRef.ValueOperand;
                state.Push(op);
            }
            else
            {
                state.Push(new ErrorValueOperand(ErrorValueType.Name));
            }
        }