Add is an overridden procedure that implements the interface used to add a socket handle to the list. 

Add use Lock to protect access to socket handles in the list during the operation, and calls Unlock prior to exiting from the method. 

Add raises an EIdStackSetSizeExceeded if the number of socket handles in the list would exceed FD_SETSIZE when the handle is added. 

Add increases the length of the socket descriptor set, and stores AHandle at the highest position in the list. 

Use Remove to delete a socket handle from the list, or Clear to remove all socket handles in the list.