  We introduce diagrams and essential sets for signed permutations, extending
the analogous notions for ordinary permutations. In particular, we show that
the essential set provides a minimal list of rank conditions defining the
Schubert variety or degeneracy locus corresponding to a signed permutation. Our
essential set is in bijection with the poset-theoretic version defined by
Reiner, Woo, and Yong, and thus gives an explicit, diagrammatic method for
computing the latter.
