Adapting an old building for modern use comes with many challenges, especially when that adaptation includes an addition. Should the addition imitate the existing structure? Should it pay homage without being identical? Should it be of a completely different style, but one that complements or, at the least, defers to its surroundings? Or should you just throw a huge folding glass thing on top and call it a day? George Boyle, the architect in charge of turning the former Mercantile Library at 17 East 47th Street into the Merc Hotel, is going with the fourth option. On his website, Boyle explains, "We sought to design a roof-top addition that speaks softly to the original structure - to converse rather than merge." Okay, these two structures are "conversing" with each other. What are they conversing about? We're not sure we want to know.