<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def deprecated(msg=None, stacklevel=2):
 """ Used to mark a function as deprecated. Parameters msg : str The message to display in the deprecation warning. stacklevel : int How far up the stack the warning needs to go, before showing the relevant calling lines. Examples -------- @deprecated(msg='function_a is deprecated! Use function_b instead.') def function_a(*args, **kwargs):
 """

    def deprecated_dec(fn):
        @wraps(fn)
        def wrapper(*args, **kwargs):
            warnings.warn(
                msg or "Function %s is deprecated." % fn.__name__,
                category=DeprecationWarning,
                stacklevel=stacklevel
            )
            return fn(*args, **kwargs)
        return wrapper
    return deprecated_dec