def trace(func):
    def wrapper(*args):
        print(' ' * wrapper.space, end = '')
        print('{} <- {}'.format(func.__name__, str(args)))
        wrapper.space += 1
        val = func(*args)
        wrapper.space -= 1
        print(' ' * wrapper.space, end = '')
        print('{} -> {}'.format(func.__name__, str(args)))
        return val
    wrapper.space = 0
    return wrapper

@trace
def puissance(a, n):
    pass



@trace
def expoR(a, n):
    pass