I N P U T S     O U T P U T S
AB  AB' LD'     LD  Action
--  --  --      --  ------
 A   A   F       F         
 A   A   B       B         
 A   B   F       F   INC   
 A   B   B       F         
 A   C   F       F   ADD 2 
 A   C   B       B   SUB 2 
 A   D   F       B         
 A   D   B       B   DEC   

 B   A   F       B         
 B   A   B       B   DEC   
 B   B   F       F         
 B   B   B       B         
 B   C   F       F   INC   
 B   C   B       F         
 B   D   F       F   ADD 2 
 B   D   B       B   SUB 2 

 C   A   F       F   ADD 2 
 C   A   B       B   SUB 2 
 C   B   F       B         
 C   B   B       B   DEC   
 C   C   F       F         
 C   C   B       B         
 C   D   F       F   INC   
 C   D   B       F         

 D   A   F       F   INC   
 D   A   B       F         
 D   B   F       F   ADD 2 
 D   B   B       B   SUB 2 
 D   C   F       B         
 D   C   B       B   DEC   
 D   D   F       F         
 D   D   B       B