docs: def diffPrefsPrior(priorstring):
      """Parses `priorstring` and returns `prior` tuple."""
      assert isinstance(priorstring, str)
      prior = priorstring.split(',')
      if len(prior) == 3 and prior[0] == 'invquadratic':
          [c1, c2] = [float(x) for x in prior[1 : ]]
          assert c1 > 0 and c2 > 0, "C1 and C2 must be > 1 for invquadratic prior"
          return ('invquadratic', c1, c2)
      else:
          raise ValueError("Invalid diffprefsprior: {0}".format(priorstring))