ble, from left to right.

This effectively reduces the iterable to a single value.  If initial is present,
it is placed before the items of the iterable in the calculation, and serves as
a default when the iterable is empty.

For example, reduce(lambda x, y: x+y, [1, 2, 3, 4, 5])
calculates ((((1 + 2) + 3) + 4) + 5).