A departure for Mr. Francis Stuart, from whom we are prone to expect rather oblique fantasy. In this he has given us rollicking farce, a neat and entertaining bit of character sketching, and Thorne-Smith-with-the-dirt-left-out comedy. Middle aged Margaret Hubbard follows in the footsteps of her dead husband, and bets a year's savings on the ponies -- and then gets cold feet and hides in the toilet room. Her encounter with a glamorous movie star ends in her landing a publicity job as ""Everybody's Mummy"" -- and a parody of the publicity-mad movie world unrolls. The wind up is as original and amusing as the balance of the story -- and it should score a hit. Sell to the people who liked the play, Once in a Lifetime -- to those who liked Phil Ston's Farmer in the Dell. Good straight humor -- of which there is too little.