// visitor pattern
 public override void Accept(ISoqlVisitor visitor)
 {
     visitor.Visit(this);
 }
 // visitor pattern
 public abstract void Accept(ISoqlVisitor visitor);
 // visitor pattern
 public override void Accept(ISoqlVisitor visitor)
 {
     visitor.Visit(this);
 }
 // visitor pattern
 public abstract void Accept(ISoqlVisitor visitor);