1. A method for determining an element corresponding to an index value for a union of a first sorted set and a second sorted set, comprising: a) selecting both a first element associated with a first sorted set sub index and a second element associated with a second sorted set sub index, a sum of the first sorted set sub index and the second sorted set sub index being equal to an index value; b) determining a lowest element of the first element and the second element; c) discarding the lowest element and preceding lower elements of a respective sorted set; d) decrementing the index value by the sub index of the respective sorted set; e) repeating operations a)-d) until the index value equals one; and when the index value reaches one, selecting a minimum remaining value as the element corresponding to the index value.