Prospects for a four-nation agreement to pull Cuban troops out of an African war were brighter Tuesday after negotiators "substantially narrowed the gap" on their differences, U.S. officials said. Although no final deal was struck, Cuban officials indicated they were willing to step up the pace of the withdrawal of 50,000 troops from Angola as well as the 16,000 to 18,000 stationed near the border with Namibia, the officials said. In exchange, South Africa would pull its troops from Namibia and prepare the territory for independence.