const valueAt = (xs, i, acc=0) => {
if (isEmpty(xs))
return null;
return (i === acc)
? head(xs)
: valueAt(tail(xs), i, acc + 1)
};
const positionOf = (xs, x, acc=0) => {
if (isEmpty(xs)) {
return -1;
}
return (head(xs) === x)
? acc
: positionOf(tail(xs), x, acc + 1);
};
const reverse = (xs, acc=[]) => (
isEmpty(xs)
? acc
: reverse(tail(xs), [head(xs)].concat(acc))
);