sprrk(a,time,param)

function deriv = sprrk(a,time,param)
%  Function to compute 3 mass-spring system
%  using Runge-Kutta integrator
%  The time is not used in this version
%  The vector a is [x(1) x(2) x(3) v(1) v(2) v(3)]
%  The vector param = k_over_m
%  The vector deriv = [dx(1)/dt dx(2)/dt ... dv(3)/dt]
deriv(1) = a(4);
deriv(2) = a(5);
deriv(3) = a(6);
param2 = -2*param;
deriv(4) = param2*a(1) + param*a(2);
deriv(5) = param2*a(2) + param*(a(1)+a(3));
deriv(6) = param2*a(3) + param*a(2);
return;