code: def lnprior(x):
      
      per, t0, b = x
      if b < -1 or b > 1:
          return -np.inf
      elif per < 7 or per > 10:
          return -np.inf
      elif t0 < 1978 or t0 > 1979:
          return -np.inf
      else:
          return 0.
docs: Return the log prior given parameter vector `x`.