Take, if you will, cannabis sativa – better known as marijuana.  It’s in the news because the states are continuing to legalize it for medical and recreational use, despite federal prohibition. But Donald Trump’s executive branch is making noise that it will begin more vigorous enforcement of federal laws prohibiting it. This is in the news at the same time as the issue of sanctuary cities, with some local and state governments saying they will not enforce federal law against undocumented immigrants. Could the states also choose not to enforce federal prohibitions against marijuana,...