I am writing a library on top of grid/foreign-array that provides math functions corresponding to the ones in the Numbers Dictionary of the hyperspec (http://www.lispworks.com/documentation/HyperSpec/Body/c_number.htm) (I will share it when it is done) For those that are interested into details, it parses declarations of the type: ... (cos ((!F :return !F) (!C :return !C))) (tan ((!F :return !F) (!C :return !C))) ... to generate generic functions and methods that will accept grid vectors: (grid-cos #1(....)) returns a grid double-float of cosines of double floats (grid-cos #2(...)) returns a grid complex double-float of cosines of complex arguments. Question: I am not sure how to store results for predicate type functions, such as minusp, zerop, and comparison functions >, = that return t or nil. Is there a foreign-array or cl-array type that stores nils and t's? Thanks, Mirko