In late August, the Pomona College Museum of Art was first out of the Pacific Standard Time gate with the lead installment of its three-part survey, "It Happened at Pomona: Art at the Edge of Los Angeles, 1969-1973." The exhibition was modest in size but surprisingly rich in implications. Not the least of its virtues, especially in our bigger-is-better era of art extravaganzas, was the vivid demonstration that a small college art museum can have a big impact if a keen intelligence is at work.