def identify_course(url):
    """ 
    Identify if the url references a course.
    If possible, returns the referenced course, otherwise returns None.
    """

    # Check for the position previous to the course
    index = url.find("sigla=")

    # Check if the position has been found and extracts the course from the string.
    # Otherwise returns None
    if index != -1:
        init_position = index + len("sigla=")
        course = url[init_position:len(url)]

        # Return statement
        return course
    
    # None statement
    return None