The European Union Summit will be held June 20-22 in Seville. Demonstrations are likely but security will be tight, especially in view of the thwarting by police of a terror attack in May in advance of a summit between European and Latin American leaders. Police found some 400 p0unds of explosives, detonators and automatic weapons in an apartment in Madrid and arrested two suspects and were in pursuit of a third. The suspects are believed to be members of ETA, the violent Basque separatist organization.