For those not in the know (I had to double-check myself), the difference between explicit and semi-implicit Euler is simply the order in which position and velocity are updated.

Explicit updates position with the old velocity:

    particle.position += particle.velocity * dt;
    particle.velocity += acceleration * dt;

Semi-implicit (symplectic) with the new velocity:

    particle.velocity += acceleration * dt;
    particle.position += particle.velocity * dt;