For simplicity we follow the rules: I, J denote sets, A, B denote many sorted sets of I, i, j, x are arbitrary, and S denotes a non empty many sorted signature. A set has common domain if: (Def.1) For all functions f , g such that f ∈ it and g ∈ it holds dom f = dom g. Let us mention that there exists a set which is functional and non empty and has common domain. The following proposition is tr...