[ Diff(x[1], t) = (-1)*k[7]*x[1]*k[2]/k[2],
  Diff(x[2], t) = ((-1)*k[5]*x[2]*k[2] + 1*k[7]*x[1]*k[2])/k[2],
  Diff(x[3], t) = (1*k[5]*x[2]*k[2] + (-1)*k[6]*x[3]*k[3])/k[3],
  Diff(x[4], t) = 1*k[6]*x[3]*k[3]/k[4] ]