  We provide a novel tool which may be used to construct new examples of
positive maps in matrix algebras (or, equivalently, entanglement witnesses). It
turns out that this can be used to prove positivity of several well known maps
(such as reduction map, generalized reduction, Robertson map, and many others).
Furthermore, we use it to construct a new family of linear maps and prove that
they are positive, indecomposable and (nd)optimal.
