% W = gramian(sys, t, t0) computes the gramian controllability matrix of
% the state space system sys. For each timestep t. If t0 is not set, the
% default value is 0. For the computation the result from
%
% C. Van Loan, "Computing integrals involving the matrix exponential," in IEEE Transactions on Automatic Control, vol. 23, no. 3, pp. 395-404, June 1978.