p53 Initial value parameter to `reduce` function silently dropped

```
function sumPrices (...numbers {
  return numbers.reduce(function(total, val) {
    return total + (Number.isNaN(Number(val))? 0 : Number(val))
  }**, 0**)

function sumPrices (...numbers {
  return numbers.reduce((total, val) => 
    total + (Number.isNaN(Number(val))? 0 : Number(val)) /* **, 0** */)
```