// example 6

<'
struct  packet_s {  
   
   length : uint; 
   keep length >= 10 and length <=20; 
   
   with_checksum : bool;    
   // This Boolean variable will be generated randomly. If it is FALSE the packet will not have a field called
   // “checksum” at the end and the LSB of the field “header1”  will be '0'. If it is TRUE a field called
   // “checksum” will be added to the packet through a WHEN inheritance and the LSB of the field
   // “header1” will be '1'.

   %header1 : byte; 
   keep header1 != 0;

   keep with_checksum == FALSE => header1[0] == '0';    
   // If the packet is without checksum the first bit of the field “header1” will be '0'
   keep with_checksum == TRUE => header1[0] == '1';
   // otherwise it will be '1'
   
   %header2 : byte;
   keep header1 <  128 => header2 != 0; 
   
   %data : list of byte;                      
   keep data.size() == length-2; 

   when TRUE'with_checksum { // When the field “with_checksum” is TRUE
      
      %checksum : byte;         // an extra field called “checksum” is added to the packet.
      keep checksum == data.xor();    
      // The value of the new field is equal to the “xor()” of all the bytes in the data list
      // You can look up the “xor()” method of a list on the e manual on the net.
   };        
};

'>