Mercedalia Diaz was tired of being an illegal immigrant, living in fear of arrest and separation from her young son. So she filed an application to work legally in the United States — in effect, turning herself in. After a permit was denied, Diaz landed here, in U.S. Immigration Court in Los Angeles, nervously fielding questions from a federal judge: If you don't get permanent residency, would you agree to leave the U.S. voluntarily? Do you promise not to return without a visa? What would you do with your son? ... Then Diaz told her story to Judge Bruce J....