installed as the trace function for debugged
frames. Its return value is the new trace function, which is
usually itself. The default implementation decides how to
dispatch a frame, depending on the type of event (passed in as a
string) that is about to be executed.

The event can be one of the following:
    line: A new line of code is going to be executed.
    call: A function is about to be called or another code block
          is entered.
    return: A function or other code block is about to return.
    exception: An exception has occurred.
    c_call: A C function is about to be called.
    c_return: A C function has returned.
    c_exception: A C function has raised an exception.

For the Python events, specialized functions (see the dispatch_*()
methods) are called.  For the C events, no action is taken.

The arg parameter depends on the previous event.
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