function v = prob1f(t,y,Z,a,b,c,d)
v = zeros(2,1);
if isempty(Z)     % ODEs
   v(1) = a * y(1) + b * y(1) * y(2);
   v(2) = c * y(2) + d * y(1) * y(2);
else              % DDEs
   m = 200;
   ylag = Z(:,1);
   v(1) = a * y(1) * (1 - y(1) / m) + b * y(1)    * y(2);
   v(2) = c * y(2)                  + d * ylag(1) * ylag(2);
end