ell_stm_ode(t, x, mydata, n, back)

function dxdt = ell_stm_ode(t, x, mydata, n, back)
%
% ELL_STM_ODE - ODE for state transition matrix.
%

  if back > 0
    t = -t;
  end
  
  A  = ell_value_extract(mydata.A, t, [n n]);
  Z  = zeros(n, n);
  AA = A;
  ZZ = Z;
  
  for i = 1:(n - 1)
    AA = [AA ZZ; ZZ' A];
    ZZ = [ZZ; Z];
  end

  if back > 0
    dxdt = -AA * x;
  else
    dxdt = AA * x;
  end

  return;