def sl(c, s, l):
    """ This accountancy function computes straight line
    depreciation for an asset purchase for cash with
    a known life span and salvage value.
    c = historical cost or price paid (1000)
    s = the expected salvage proceeds at disposal
    l = expected useful life of the fixed asset
    Example: sl(1000, 350, 10)
    """
    return (c - s) / l
