Oh it's very simple. The operators + - * / < > <= >= are the only infix operators (for now). They have the usual precedence rules that other languages use.
How they work is, they take one expression on the left, and one expression on the right, and then wrap em in a list, so that `X + Y` becomes `(add X Y)`, and then "add" is the actual add function.
So they're just syntax sugar for common infix operations, that's all. That's why the last example passed "mul" to "sum" rather than "*".
