WE HAVE been suspicious for some time of laws imposing mandatory minimum sentences. Extending prison time for criminals who abuse children may be more justifiable than mandatory minimums for non-violent drug offences, but it seems that lawmakers were not particularly careful when, in the 1990s, they sat down to compose these tougher laws. A case in point: 18 U.S.C. § 2252(b)(2), a provision of the federal criminal code prescribing a 10-20 year prison sentence for anyone violating a child-pornography law who has also been convicted previously under state laws “relating to aggravated sexual abuse, sexual abuse, or abusive sexual conduct involving a minor or ward”. Avondale Lockhart was caught buying child pornography in 2010 and pleaded guilty to the charges. But when his prison sentence was extended from an initial range of 78-97 months to the mandatory minimum of 10 years because he had been convicted years earlier of trying to rape his girlfriend, he cried foul. His prior conviction did not involve “a minor or a ward”, he said, so it does not trigger the longer sentence.

Well, let’s read the law again. The three classes of crimes that count are “aggravated sexual abuse, sexual abuse, or abusive sexual conduct involving a minor or ward.” Do the final five words refer just to the third item in the list (“abusive sexual conduct”)? Or do they modify, as Mr Lockhart claims, all three categories of sex crimes? Leaving aside the apparent redundancies among these terms, the niggling grammatical point makes a world of difference. If the qualifier applies only to “abusive sexual conduct”, then Mr Lockhart’s attempted rape of an adult counts against him and will buy him a longer sentence. If the qualifier applies to the whole line, it doesn’t, since he did not attempt to rape a child.

When the Supreme Court took up the dicey interpretive question in Lockhart v United States on November 3rd, the justices chastised Congress for sloppy prose. “[W]e give them a ‘D’ for their drafting of this statute,” Justice Samuel Alito said, generously sparing lawmakers an F. Had Congress wanted to clearly convey Mr Lockhart’s meaning, they could have repeated “involving a minor or ward” after each of the three crimes. Or, less clunkily, they could have written something like: “conduct involving a minor or ward that involves aggravated sexual abuse, sexual abuse, or abusive sexual conduct.” Likewise, Congress could have limited the modifier to the final crime by using semi-colons, rather than commas, between the items in the series. There were any number of edits to the statute that would have precluded Tuesday’s Talmudic disputation at One First Street, NE.

But no such luck: the justices had to do their best to interpret the messy law that Congress passed. Justice Antonin Scalia wrote the book, literally, on so-called “canons” of statutory interpretation that judges turn to when trying to ascertain the meaning of a law. Two commonly used heuristics are at war in Lockhart. The “last antecedent” canon says that a phrase like “involving a minor or ward” should “ordinarily be read as modifying only the noun or phrase that it immediately follows.” Meanwhile, according to the “series-qualifier” canon, the qualifying phrase applies to every item in the series as long as their nouns and verbs are laid out in parallel.

In other words, who knows. In the oral argument, Justice Elena Kagan said, “I showed it to all my clerks and basically said how would you read it. And two came out on one side, and two came out on the other side.” Justice Scalia was similarly ambivalent: ”I have no assurance what the right answer is”, he said to Ann O’Connell, who argued the government’s case. There is enough ambiguity in the law, Justice Scalia said, that the “rule of lenity”—a tie-breaker that errs on the side of a lighter sentence when a statute can easily be read in two ways—may point toward a win for Mr Lockhart.

In a series of questions late in the hearing, Justice Scalia and Justice Stephen Breyer used Ms O’Connell as a foil for another episode in their old spat over whether the best way to read a law is by focusing squarely on the text and ignoring congressional intent (Mr Scalia’s view) or by considering evidence of what lawmakers had in mind when their staffs drafted and they enacted a law (Mr Breyer’s). When an interpretive tangle leaves two readings “at equipoise”, Justice Breyer said, “legislative history helps”. And in a 1998 letter to the House Judiciary Committee, Mr Lockhart’s lawyer argued in his brief, the Department of Justice wrote that the point of the law was to enhance the punishments of criminals with “prior state convictions for child molestation” (emphasis added), not for convictions involving sex crimes against adults.

The upshot? Mr Lockhart seems likely to get the vote of a strong majority of the Court, though probably for several different reasons. One hopes the justices’ ruling will be more clearly written than the law they are struggling to decipher.