“ It is the position of the Attorney General that petitioner's failure to appeal in this case barred him from seeking post-conviction relief by way of a collateral habeas corpus proceeding. He admits that habeas corpus is available in California in cases involving certain exceptional circumstances, but contends that this is not such a case. If the Attorney General is correct, the judgment may rest on a non-federal ground, thus calling for dismissal of our writ of certiorari. In this state of uncertainty, we follow our precedents in Herb v. ”