In this paper I explore the possibility of introducing in engineering ontologies a generic part-whole relation for functions of technical artefacts by functional composition or functional decomposition. I show by means of the postulates of mereology that general functional compositions and decompositions cannot define such a relation. Yet, one can argue that functional decompositions that are acceptable in engineering may define a proper generic part-whole relation for functions. This possibility requires that (i) the part-whole relation is relative to the specific organisation of functions in decompositions, (ii) there is no strict symmetry between functional composition and decomposition, and (iii) functional decomposition is not transitive.