Sum of Digits - Walkthrough

Let me trace sum_of_digits($1234$) step by step:

Call $1$: sum_of_digits($1234$) $\rightarrow$ returns $4 +$ sum_of_digits($123$)

Call $2$: sum_of_digits($123$) $\rightarrow$ returns $3 +$ sum_of_digits($12$)

Call $3$: sum_of_digits($12$) $\rightarrow$ returns $2 +$ sum_of_digits($1$)

Call $4$: sum_of_digits($1$) hits base case, returns $1$ Then the results come back: $2 + 1 = 3$, then $3 + 3 = 6$, then $4 + 6 = 10$. Four calls total, each peeling off one digit.