public virtual void VisitSnippetExpression(ISnippetExpression value)
 {
 }
 private void WriteSnippetExpression(ISnippetExpression expression, IFormatter formatter)
 {
     formatter.WriteComment(expression.Value);
 }
 public virtual IExpression TransformSnippetExpression(ISnippetExpression value)
 {
     return value;
 }