A number of ancient civilizations, including the Thracians, ancient Greeks, Scythians, Celts, ancient Romans, Goths (Ostrogoths and Visigoths), Slavs (East and West Slavs), Varangians and the Bulgars have left their mark on the culture, history and heritage of Bulgaria. Due to this great variety of influences, Bulgaria has adopted many unusual traditions. Thracian artifacts include numerous temples, tombs, golden treasures and ancient rites and rituals, while the Bulgars have left traces of their heritage in statehood, early architecture, music and dances. Thracian rituals such as the Tryphon Zarezan which is dedicated to Saint Tryphon of Campsada, Kukeri and Martenitsa are to this day kept alive in the modern Bulgarian culture. The oldest treasure of worked gold in the world, dating back to the 5th millennium BC, comes from the site of the Varna Necropolis. Bulgaria functioned as a cultural hub of Slavic Europe during much of the Middle Ages, exerting considerable literary and cultural influence over the Eastern Orthodox Slavic world by means of the Preslav and Ohrid Literary Schools. Bulgaria also gave the world the Cyrillic script, the second most widely used alphabet and sixth most-used writing system in the world, which originated in these two schools in the tenth century. Bulgaria's contribution to humanity continued throughout the nineteenth and twentieth centuries, with individuals such as John Atanasoff — a United States citizen of Bulgarian and British descent, regarded as the father of the digital computer. A number of noted opera-singers (Nicolai Ghiaurov, Boris Christoff, Raina Kabaivanska, Ghena Dimitrova, Anna Tomowa-Sintow, Vesselina Kasarova), pianist Alexis Weissenberg, and successful artists (Christo, Pascin, Vladimir Dimitrov) popularized the culture of Bulgaria abroad. Music Bulgaria has a long-standing musical tradition dating to the early Middle ages. One of the earliest known composers of Medieval Europe, Yoan Kukuzel (ca. 1280–1360), became famous for his work Polieleion of the Bulgarian Woman. About 90 of his works have survived. Kukuzel also reformed the Byzantine musical writing system, and became known as The Angel-voiced for his singing abilities. The tradition of church singing in Bulgaria is more than thousand years old. In the Bulgarian Orthodox Church there are two traditions of church singing - Eastern monodic (one-voice) singing and choral (polyphonic). The Eastern monodic singing observes the tradition of Greek and Byzantine music as well as the requirements of the eight-voices polyphonic canon of the Eastern Orthodox Church. The second tradition is the choral church music, established