Stereoselective [π4s+π2s]-dipolar cycloaddition of pyridinium ylides to nitriles of the trans-1,3-butadiene series. Crystal and molecular structure of 3-benzoyl-1-(2,2-dicyano-1-cyclopropylvinyl)-2-phenyl-6-cyano-1,2-trans-2,3-trans-1,9-trans-1,2,3,9-tetrahydroindolizine