Geometric Numerical Integration implicit method and explicit method?

dt = 0.02; t = 0.0:dt:4000.0; p = zeros(size(t)); q = zeros(size(t)); p(1)= 0; q(1)= 1; for ii = 1:(length(t)-1) p(ii+1)= p(ii) - dt*(-sin(q(ii))); q(ii+1)= q(ii) + dt*(p(ii)); end quiver(t,p,t,q)