Parser: chained equivalence
Translate a chain of equivalences A <-> B <-> C into a conjunction (A <-> B) /\ (B <-> C). Implication is weaker than equivalence when it occurs to the left of it, and is forbidden at the right hand side. In other words, A -> B <-> C <-> D is allowed and translated into A -> ((B <-> C) /\ (C <-> D)), and A <-> B -> C is not allowed, and requires explicit parentheses.
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