Synthesis of 2,2′-Ethylene-bis(1,2-dihydrobenzo(h)-3H-4,2-benzoxazine) and 3,3′-Ethylene-bis(3,4-dihydrobenzo(h)-2H-1,3-benzoxazine) and Assignation of Their 1H-NMR Spectra Using the LAOCN3 Computer Program.