I should be able to fold over Lists

(User Story, Closed -> Fixed, Priority: Critical, Test Status: Unit tested , Reported By Justin du Coeur, )
Functions
Summary: Specifically, I need to be able to sum a List. But let's implement the more powerful general case first.
This should use foldLeft() under the hood, for stack safety.
It should probably bind $acc and $next as the general mechanism for defining the function.