text:  It's Morning Edition from NPR News. I'm Steve Inskeep. I'm Leila Faldin. And I'm A. Martinez. On this Fourth of July, we bring you a story about a celebration of American history. One of the world's largest murals is painted on the side of a concrete-lined river running right through the city of Los Angeles. Among the dozens of artists who have left their mark on the Great Wall of Los Angeles are Ricardo Mendoza and Rio Diaz.