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*Proposition:*
For all formulas `A` and `B`, the following holds:

A<=>Biff B<=>AA<=>Biff ( A/\B) \/ (~A/\ ~B)A<=>Biff ( A=>B) /\ (B=>A)

*Equivalence can thus be defined by implication and conjunction.*

*Proposition:* Equivalence is not associative.

*Proof:* by construction of a counterexample.

Author: Wolfgang Schreiner

Last Modification: October 6, 1999