Recursion: deep sum

A recursive function calls itself on a smaller piece of the problem. Every recursion needs a base case that stops it:

def countdown(n):
    if n == 0:
        return "liftoff"        # base case: stop recursing
    return countdown(n - 1)     # recurse on a smaller n

For nested data, the base-vs-recurse test is usually a type check: isinstance(x, list) asks "is this a list?". That lets one function handle a value that might be a plain number or a nested list of numbers, at any depth — recurse when there's more structure to dig into, stop when there isn't.

Your task

Write deep_sum(x) where x is an int or a (possibly nested) list of ints/lists. Return the sum of every int at any depth.

deep_sum([1, [2, [3, 4]], 5])   # 15
deep_sum([])                    # 0
deep_sum(7)                     # 7

Hint: two cases. When x is a list, the answer is the combined deep_sum of each item inside it; otherwise x is a plain int that already stands for its own total.