{ diff(x1, t) = 0/k23,
  diff(x2, t) = (1*1/k2*(1 - k6)*x1 + (-1)*1/k2*x2 + (-1)*k4*(365/k13 + 1/k2)*x2*(x3 + x10)/x1 + (-1)*k4*(365/k13 + 1/k2)*x2*(x4 + x9 + x8)/x1 + 1*1/k14*x5 + 1*1/k14*x6 + 1*1/k14*x11 + 1*1/k15*x7)/k23,
  diff(x3, t) = ((-1)*1/k2*x3 + 1*k4*(365/k13 + 1/k2)*x2*(x3 + x10)/x1 + (-1)*365/k13*x3)/k23,
  diff(x4, t) = ((-1)*1/k2*x4 + 1*k4*(365/k13 + 1/k2)*x2*(x4 + x9 + x8)/x1 + (-1)*365/k13*x4)/k23,
  diff(x5, t) = ((-1)*1/k2*x5 + 1*365/k13*x3 + (-1)*k4*(365/k13 + 1/k2)*(1 - k8)*x5*(x4 + x9 + x8)/x1 + (-1)*1/k14*x5)/k23,
  diff(x6, t) = ((-1)*1/k2*x6 + 1*365/k13*x4 + (-1)*k4*(365/k13 + 1/k2)*(1 - k8)*x6*(x3 + x10)/x1 + (-1)*1/k14*x6)/k23,
  diff(x7, t) = (1*1/k2*k6*x1 + (-1)*1/k2*x7 + (-1)*k4*(365/k13 + 1/k2)*(1 - k7)*x7*(x4 + x9 + x8)/x1 + (-1)*1/k15*x7)/k23,
  diff(x8, t) = ((-1)*1/k2*x8 + 1*k4*(365/k13 + 1/k2)*(1 - k7)*x7*(x4 + x9 + x8)/x1 + (-1)*365/k13/(1 - k10)*x8)/k23,
  diff(x9, t) = ((-1)*1/k2*x9 + 1*k4*(365/k13 + 1/k2)*(1 - k8)*x5*(x4 + x9 + x8)/x1 + (-1)*365/k13/(1 - k9)*x9)/k23,
  diff(x10, t) = ((-1)*1/k2*x10 + 1*k4*(365/k13 + 1/k2)*(1 - k8)*x6*(x3 + x10)/x1 + (-1)*365/k13/(1 - k9)*x10)/k23,
  diff(x11, t) = ((-1)*1/k2*x11 + 1*365/k13/(1 - k9)*x10 + 1*365/k13/(1 - k9)*x9 + 1*365/k13/(1 - k10)*x8 + (-1)*1/k14*x11)/k23 }