How does peeling off L[0] and trusting the function you haven't finished writing turn summing a list into a loop-free recursive step?
Short drills on what this video just taught. Write the code, run the checks, and reveal the answer only if you are stuck.
total_recur peels the first element off the front and hands the rest of the list to another call. The shipped version stops as soon as the list it is handed is empty.
Start it on [8, 3, 5] and trace it on paper. Write down the value of L for every call that happens — beginning with the whole starting list and ending with the list that arrives at the call which peels nothing off at all. Assign that sequence to calls as a list of lists, in the order the calls happen.