©1992-2000 Colin Allen. Reproduction by any means strictly prohibited.

The text book says:
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intro
Given two conditional sentences having the forms Φ → Ψ and Ψ→Φ (at lines m and n), conclude a biconditional with Φ on one side and Ψ on the other.
 Annotation: m, n ↔I
 Assumption set:The union of the assumption sets at lines m and n.
 Comment: The order of m and n in the proof is irrelevant.
 
 
 Examples.
 
1       (1)     P→Q                  A
 
2       (2)     Q→P                  A
 
1,2     (3)     P↔ Q               1,2 ↔I
 
or
1,2     (3)     Q ↔ P               1,2 ↔I