Exactly, that was my take-away. You don't need to think of the Sum function as a for-loop every time you use Sum, but it certainly helps explain it to a program

Exactly, that was my take-away. You don't need to think of the Sum function as a for-loop every time you use Sum, but it certainly helps explain it to a programmer who doesn't have a sense of what Sum is, but knows what a for-loop is.