{ diff(x1, t) = (1*k18*k1*x2*x1*(1 - x1/k2) + (-1)*k18*k5*x1 + (-1)*k18*k3*x3*x1)/k18,
  diff(x2, t) = ((-1)*k18*k10*x2 + (-1)*k18*k9*x1*x2 + (-1)*k18*(k13 + k7*(k8 + x3))*x2 + 1*k18*k10*x4)/k18,
  diff(x3, t) = (1*k18*k7*(k8 + x3)*x2 + 1*k18*k4*x1*x3 + (-1)*k18*k6*x3 + (-1)*k18*k14*x1*x3)/k18,
  diff(x4, t) = (1*k18*k10*x2 + 1*k18*piecewise(k12*sin(6*pi*t), k11 < k12*sin(6*pi*t), k11) + (-1)*k18*k13*x4 + (-1)*k18*k10*x4)/k18 }