dimension problem = {ex3a, ex3b, ex3c, ex3d}

param lambda[problem] = {1.5, 2.0, 2.5, 3.0}

state y[problem] = 0.0 delay

aux ylag[problem] = (t>1)?y[problem](t-1):t-1
y' = -lambda*ylag*(1 + y)

print t, y