Solved by ptrblck in post #4 
              
              
                The underscore (_) is a special variable which will get the output of the last operation as seen here: 
a = 5

a
Out[2]: 5

6
Out[3]: 6

_
Out[4]: 6

_ + _
Out[5]: 12

If you are stepping through the code in a REPL env I would guess that in particular this variable can hold a reference to the CUDATe…