Although its script is a great deal funnier than you might be expecting, the profound senses of heartbreak and loss that fuel David Lindsay-Abaire's Rabbit Hole tend to sneak up on you and hit like waves, knocking you off balance and leaving you somewhat shaken. Anyone attending the Richmond Hill Barn Theatre's current presentation of the playwright's family drama is advised to bring tissues. (Unless you go the route I did, and surreptitiously dry your cheeks during scene-change blackouts.) Yet there's something else you might also want to bring, something I hadn't anticipated through a mere reading of this Pulitzer Prize-winner: a bib.