PRESERVING Boston Common should mean much more than keeping the grass green ("Large events may become uncommon at city's beloved park," Page B1, March 20). The Common was America 's very first public grounds and the city should not regulate away the right of the people to assemble peaceably there for the sake of a greener lawn. If greener grass is what the city wants, they can achieve that by not allowing dogs to urinate on the grounds. Boston Parks Department maintenance people have told anyone who asks that dog urine, not peaceable assemblies, is what damages the grass most.