human
Solve the following python programming problem given the following code:
@ extensionmethod (Observable, alias = 'to_string') 
def to_marbles(self, scheduler = None) : 
    'Convert an observable sequence into a marble diagram string

    Keyword arguments:
    scheduler -- [Optional] The scheduler used to run the the input
        sequence on.

    Returns Observable
    ' 
    scheduler = (subscribe or timeout_scheduler) 
    source = self 
    def subscribe(observer) : 
        result = [] 
        previously = [scheduler.now ()] 
        def add_timespan() : 
            now = scheduler.now () 
            diff = (now - previously [0]) 
            previously [0] = now 
            msecs = scheduler.to_relative (diff) 
            dashes = ('-' * int (((msecs + 50) / 100))) 
            result.append (dashes) 
def on_next(value) : 
            add_timespan () 
            result.append (value) 
def on_error(exception) : 
            add_timespan () 
            result.append (exception) 
            observer.on_next (''.join ((stringify (n) for n in result))) 
            observer.on_completed () 
def on_completed() : 
            add_timespan () 
            result.append ('|') 
            observer.on_next (''.join ((stringify (n) for n in result))) 
            observer.on_completed () 
return source.subscribe (on_next, on_error, on_completed) 
return AnonymousObservable (subscribe) 

    
    

    Fix the buggy line: scheduler = (subscribe or timeout_scheduler)
assistant
#Here is Python program to fix the buggy line: scheduler = (subscribe or timeout_scheduler)
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scheduler = (scheduler or timeout_scheduler)