 A set of truth-functions is strongly functionally complete if every truth-function can be obtained by composition from functions in the set, together with the projection functions, and weakly functionally complete if every truth-function can be obtained from functions in the set, together with the projection functions and the operation of passing from an (n+1)-ary function to an n-ary function by dropping an inessential argument (an argument on which the function does not depend, that is). This terminology is transferred from truth-functions to Boolean connectives via the conventional association of such connectives with truth-functions described in the previous paragraph. Thus for example each of the sets {∧,¬}, {→,¬} is weakly functionally complete, while {→,⊥} is strongly functionally complete. This weak/strong distinction is not usually registered in the literature, and functional completeness simpliciter is understood as weak functional completeness. These notions also apply in matrix semantics generally (see Section  5),  and there are analogous notions of expressive completeness tailored to other areas; two distinct versions of expressive completeness in the area of intuitionistic logic (the distinction being orthogonal to the weak/strong distinction just drawn) are contrasted in Rousseau (1968), for example. None of these matters will occupy us further here.