def sum_list(L: list) -> float:
    """
    Return sum of the numbers in possibly nested list L
    
    >>> sum_list([1, 2, 3])
    6
    >>> sum_list([1, [2, 3, [4]], 5])
    15
    """
    
    return sum([sum_list(x) if isinstance(x, list) else x for x in L])