# Add integrator parameter to VGPU
class VGPU:
    def __init__(self, num_trajectories, latent_dim=1,
                 num_timesteps=30, dt=0.5, lam=1.0,
                 integrator='rk4'):   # new
        ...
        self.integrator = integrator

    def _integrate_one(self, h0):
        ...
        for step in range(self.T):
            t = step * self.dt
            if self.integrator == 'euler':
                h = h + self.dt * self._safe_F(h, t, h0)
            else:
                # existing RK4 code
        ...

# In matmul demo, set integrator='euler'
def vgpu_matmul_element(A, B, num_steps_per_k=10, dt=0.1):
    ...
            vgpu = VGPU(..., integrator='euler')