A family joke that should have stayed at home. Up at their summer place, a converted carriage house, the Bensons find Tim Dwiggins' gravestone, and whenever they find something after that--a coachman's hat, a weathervane, sleigh bells, they relate it to the deceased coachman. At the end one youngster accidentally uncovers some old racing trophies. . . and the news that their Tim was a horse. ""If we're lucky. . . we'll be back next year to look for more secrets."" Riddle-culous, and the comic strip expressions don't endear.