Topos theorists and people in denotational semantics use the notation [[t]] to denote a mathematical object that is the meaning of t in some semantics. In particular, in the case of a formula f, [[f]] is the truth value. APL and Donald Knuth use [f] for the truth value of a formula (or so I have been told). Does anyone know who used the double square bracket notation first (topos theorists, denotational semantics people, or whoever), and whether it was suggested by the APL usage? -- Charles Wells Department of Mathematics, Case Western Reserve University University Circle, Cleveland, OH 44106-7058, USA 216 368 2893 ==============================================================================