I also think "if and only if" is only surpassed by "iff" in badness of notation, though I'd like to put in a good word for classic '='. There's always some impl

I also think "if and only if" is only surpassed by "iff" in badness of notation, though I'd like to put in a good word for classic '='. There's always some implicit inference being made about what kind of equality we're considering, and calling two formulas of logic "equal" if they have the same (truth) value in all cases is reasonable in my book. https://en.wikipedia.org/wiki/Logical_biconditional https://en.wikipedia.org/wiki/Logical_biconditional has some nice comparisons of notation, including a mention that my old friend George Boole also used '='.