public object VisitErrorExpression(ErrorExpressionAst errorExpressionAst)
 {
     // TODO: why is there no public constructor for ast with nested asts?
     return(errorExpressionAst.Copy());
 }
 public virtual ExpressionAst VisitErrorExpression(ErrorExpressionAst errorExpressionAst)
 {
     return((ErrorExpressionAst)errorExpressionAst.Copy());
 }