def get_essential( m ):
    """ Get the "essential" leds, along with the associated tri. 
        Format: 'ess[led] = tri'
        An essential LED is an LED that lights a triangle surface, where that LED is the only LED to light that triangle surface.
        i.e. without that LED, the given triangle will never be lit.
    """
    ess = {}
    for k in m:
        if len(m[k]) == 1:
            led = m[k][0]
            ess[led] = k        #ess[led] = tri
    return ess