A PHOTOISOMER OF 1,3,6,8-TETRAPHENYLCYCLOOCTATETRAENE AS A 1-1 MIXTURE OF 1,2,4,7-TETRAPHENYLCYCLOOCTATETRAENE AND 2,5,7,8-TETRAPHENYLBICYCLO(4.2.0)OCTATRIENE IN DYNAMIC EQUILIBRIUM. A STRUCTURAL ENIGMA