Fix bug in latex-printing Equivalent objects

Latex-printing an Equivalent object would only print out two
arguments, even if there are more.

It might be argued that `a≡b≡c≡d` is not the right way to print out `Equivalent(a, b, c, d)`, since what it means is that an even number of them are true.  but that's how Unicode pretty-printing has been printing it out, so I followed that model.  (Also handled the Not(Equivalent()) case, with ≢)
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