<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def log(arg, base=None):
 """ Perform the logarithm using a specified base Parameters base : number, default None If None, base e is used Returns ------- logarithm : double type """

    op = ops.Log(arg, base)
    return op.to_expr()