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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 check_for_executable(supernova_args, env_vars):
 """ It's possible that a user might set their custom executable via an environment variable. If we detect one, we should add it to supernova's arguments ONLY IF an executable wasn't set on the command line. The command line executable must take priority. """

    exe = supernova_args.get('executable', 'default')
    if exe != 'default':
        return supernova_args
    if 'OS_EXECUTABLE' in env_vars.keys():
        supernova_args['executable'] = env_vars['OS_EXECUTABLE']
        return supernova_args
    supernova_args['executable'] = 'nova'
    return supernova_args