Electronic control of stereoselectivity. 7. Stereospecificity of N-methyltriazolinedione cycloaddition to tricyclo[5.2.1.02,6]deca-2,5-diene, tricyclo[5.2.1.02,6]deca-2,5,8-triene, and tricyclo[5.2.2.02,6]undeca-2,5,8-triene