/// Use `apply` to wrap variadic functions.  `id1(f)` returns a
/// function that works just like `f` (if `f` doesn't use `this`),
/// but only when it's called with one argument.  `id2(f)` works
/// just like `f`, but only when it's called with two arguments.
/// `idn` works with any number of arguments.
function id1(fn) {
    return function(x) {
        return fn(x);
    }
}

function id2(fn) {
    return function(x, y) {
        return fn(x, y);
    }
}

function idn(fn) {
    return function() {
        return fn.apply(this, arguments);
    }
}