Sum of a Nested List

A nested list holds numbers and other lists, which can hold more lists, to any depth. Write nested_sum(items) that returns the total of every number inside items, however deeply it is buried.

Examples

nested_sum([1, [2, 3], [4, [5, [6]]]])  # returns 21
nested_sum([])                          # returns 0
nested_sum([[[]], [1.5, [2.5]]])        # returns 4.0

In the first example the numbers are 1, 2, 3, 4, 5 and 6, which add up to 21.

Note

Empty lists add nothing, so nested_sum([]) is 0. The numbers can be integers or floats.