<Li> 909 World history </Li> <Li> 910 Geography & travel <Ul> <Li> 910 Geography & travel </Li> <Li> 911 Historical geography </Li> <Li> 912 Graphic representations of surface of earth and of extraterrestrial worlds </Li> <Li> 913 Geography of & travel in ancient world </Li> <Li> 914 Geography of & travel in Europe </Li> <Li> 915 Geography of & travel in Asia </Li> <Li> 916 Geography of & travel in Africa </Li> <Li> 917 Geography of & travel in North America </Li> <Li> 918 Geography of & travel in South America </Li> <Li> 919 Geography of & travel in Australasia, Pacific Ocean islands, Atlantic Ocean islands, Arctic islands, Antarctica, & on extraterrestrial worlds </Li> </Ul> </Li> <Ul> <Li> 910 Geography & travel </Li> <Li> 911 Historical geography </Li> <Li> 912 Graphic representations of surface of earth and of extraterrestrial worlds </Li> <Li> 913 Geography of & travel in ancient world </Li> <Li> 914 Geography of & travel in Europe </Li> <Li> 915 Geography of & travel in Asia </Li> <Li> 916 Geography of & travel in Africa </Li> <Li> 917 Geography of & travel in North America </Li> <Li> 918 Geography of & travel in South America </Li> <Li> 919 Geography of & travel in Australasia, Pacific Ocean islands, Atlantic Ocean islands, Arctic islands, Antarctica, & on extraterrestrial worlds </Li> </Ul> <Li> 910 Geography & travel </Li>

What are the categories of the dewey decimal system