def sum(sequence, total=0):
    """sum(sequence, start=0) -> value

Returns the sum of a sequence of numbers (NOT strings) plus the value
of parameter 'start'.  When the sequence is empty, returns start."""
    # must forbid "summing" strings, per specs of built-in 'sum'
    if isinstance(total, str): raise TypeError
    for item in sequence:
        total = total + item
    return total