piggy 
              back
            Something 
              that is piggy-backed is something that is riding on the back of 
              something else. To piggy back (or 
              to go on a piggyback ride) is to ride on someone's back. This expression 
              is used in several contexts, not least in electrical engineering, 
              computer piggy back engineering, computer science and structural 
              geology. 
                
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