protected override Expression VisitChildren(ExpressionVisitor visitor)
 {
     // The default implementation of VisitChildren falls over
     // if the Expression is not reducible. Short-circuit that
     // with this:
     return(this);
 }
 protected override Expression VisitChildren(ExpressionVisitor visitor)
 {
     // See CommentExpression for why this is necessary:
     return(this);
 }