An inmate serving time for the murder of two men and the attempted murder of a third received the go-ahead to try to create constitutional precedent.In a written decision last week, U.S. District Judge Robert M. Dow Jr. denied Cedric Cal’s petition for a writ of habeas corpus.But Dow granted a certificate of appealability that will allow Cal to pursue a question the courts have left open — whether a claim that the petitioner is actually innocent is enough, by itself, to support a writ in a noncapital case.A …