public Predicate In(Predicate subquery)
 {
     return new In(name, subquery);
 }
 /// <summary>
 /// Logical OR constructor
 /// </summary>
 /// <param name="left">left condition</param>
 /// <param name="right">right condition</param>
 public Or(Predicate left, Predicate right)
 {
     this.left = left;
     this.right = right;
 }
 /// <summary>
 /// Constructor of In operator
 /// </summary>
 /// <param name="name">attribute name</param>
 /// <param name="subquery">nested subquery</param>
 public In(string name, Predicate subquery)
 {
     this.name = name;
     this.subquery = subquery;
 }