Note:
With "Material Equivalence" you can replace
p ≡ q by (p ⊃ q) & (q ⊃ p)
1. A v B
2. A ≡ (C & D)
3. B ⊃ (D & G) /D
| 4. ~D Assumption for Indirect Proof
| 5. [A ⊃ (C & D)] & [(C & D) ⊃ A) 2, Material Equivalence
| 6. ~D v ~C 4, Addition
| 7. ~C v ~D 6, Commutation
| 8. ~(C & D) 7, DeMorgan's law
| 9. A ⊃ (C & D) 5, Simplification
|10. ~A 9,8, Modus tollens
|11. B 1,10, Disjunctive Syllogism
|12. D & G 3,11, Modus ponens
|13. D 12, Simplification
|14. D & ~D 13,4, Conjunction
15. D Lines 4-14 Indirect Proof
Edwin