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Would it also be useful to have an operation (bitwise-= a ...) in which bit i is set in the result if bit i is the same in all the inputs? You can't get this function by folding, since you have to propagate three states for each bit: consensus 1, consensus 0, and no consensus. (I sort of expected eqv to behave this way, but then realized that that doesn't fit in at all to Olin's agenda of providing names for all the two-input boolean functions.)