  A minimal (by inclusion) generating set for the algebra of semi-invariants of
a quiver of dimension (2,...,2) is established over an infinite field of
arbitrary characteristic. The mentioned generating set consists of the
determinants of generic matrices and the traces of tree paths of pairwise
different multidegrees, where in the case of characteristic different from two
we take only admissible paths. As a consequence, we describe relations modulo
decomposable semi-invariants.
