fmod BOOLEAN is
        sort Boolean .

        op true : -> Boolean [ctor] .
        op false : -> Boolean [ctor] .

        op not : Boolean -> Boolean .
        op _and_ : Boolean Boolean -> Boolean .
        op _or_ : Boolean Boolean -> Boolean .

        var A : Boolean .

        eq not(true) = false .
        eq not(false) = true .
        eq true and A = A .
        eq false and A = false .
        eq true or A = true .
        eq false or A = A .
endfm