def add(a, b):
    c = []
    carry = 0

    i = 0
    while i < len(a) or i < len(b):
        if i < len(a):
            carry += a[i]
        if i < len(b):
            carry += b[i]

        ____________________

        i += 1

    if carry:
        c.append(carry)

    return c