Original sentence: In contrast to Elo and Sonas's systems, Raymond Keene and Nathan Divinsky's book Warriors of the Mind attempts to be a rating system which claims to directly compare the strength of players active in different eras and so determine the strongest player of all time
Edited sentence: In contrast to Elo and Sonas's systems, Raymond Keene and Nathan Divinsky's book Warriors of the Mind is a rare example of a rating system which claims to directly compare the strength of players active in different eras and so determine the strongest player of all time