  In this paper we formulate and prove a general theorem of stability of
exactness properties under the pro-completion, which unifies several such
theorems in the literature and gives many more. The theorem depends on a formal
approach to exactness properties proposed in this paper, which is based on the
theory of sketches. Our stability theorem has applications in proving theorems
that establish links between exactness properties, as well as in establishing
embedding (representation) theorems for classes of categories defined by
exactness properties.
