Trace calls made to the decorated function

trace_fibonacci.py

def trace():
    """
    Trace calls made to the decorated function.

    @trace("____")
    def fib(n):
        ....

    >>> fib(3)
     --> fib(3)
    ____ --> fib(2)
    ________ --> fib(1)
    ________ <-- fib(1) == 1
    ________ --> fib(0)
    ________ <-- fib(0) == 1
    ____ <-- fib(2) == 2
    ____ --> fib(1)
    ____ <-- fib(1) == 1
     <-- fib(3) == 3

    """