  We introduce a new variant of zero forcing - signed zero forcing. The
classical zero forcing number provides an upper bound on the maximum nullity of
a matrix with a given graph (i.e. zero-nonzero pattern). Our new variant
provides an analo- gous bound for the maximum nullity of a matrix with a given
sign pattern. This allows us to compute, for instance, the maximum nullity of a
Z-matrix whose graph is L(K_{n}), the line graph of a clique.
