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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:
 def backward_word(event):
 """ Move back to the start of the current or previous word. Words are composed of letters and digits. """

    buff = event.current_buffer
    pos = buff.document.find_previous_word_beginning(count=event.arg)

    if pos:
        buff.cursor_position += pos