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2. Nebular Abundance Errors.
3. The [ITAL]Forbidden[/ITAL] Abundance of Oxygen in the Sun
4. Aller, L. H. & Liller, W. 1968, in Nebulae and Interstellar Matter, ed. B. M. Middlehurst & L. H. Aller (Chicago: Univ. Chicago Press), 498
5. The ionization structure of planetary nebulae. III - NGC 7009