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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 signal(signal=None):
    '''
    Signals catalina to start, stop, securestart, forcestop.

    CLI Example:

    .. code-block:: bash

        salt '*' tomcat.signal start
    '''
    valid_signals = {'forcestop': 'stop -force',
                     'securestart': 'start -security',
                     'start': 'start',
                     'stop': 'stop'}

    if signal not in valid_signals:
        return

    cmd = '{0}/bin/catalina.sh {1}'.format(
        __catalina_home(), valid_signals[signal]
    )
    __salt__['cmd.run'](cmd)