pose of the matrix K), A is
  an N-by-N matrix in real Schur form, op(B) is an M-by-N matrix,
  U is an upper triangular matrix containing the Cholesky factor of
  the solution matrix X, X = op(U)'*op(U), and scale is an output
  scale factor, set less than or equal to 1 to avoid overflow in X.
  If matrix B has full rank then the solution matrix X will be
  positive-definite and hence the Cholesky factor U will be
  nonsingular, but if B is rank deficient then X may only be
  positive semi-definite and U w