extras⚓︎ Extra functionality (not necessary for the EnKF or the particle filter). d2x_dtdx(x) ⚓︎ Tangent linear model (TLM). I.e. the Jacobian of dxdt(x). dstep_dx(x, t, dt) ⚓︎ Compute resolvent (propagator) of the TLM. I.e. the Jacobian of step(x).