Roger Williams Park holds its ponds like a set of mirrors, and all summer the neighborhoods around it stay green in that broad-leafed, city-shaded way. Frontline Lawn Services mows through it — cutting, trimming, edging and clearing clippings on Providence lawns from Elmwood outward. Shade, tree roots and small odd-shaped lots make city mowing its own discipline; the machine has to fit the yard rather than the other way around. Done right, a lawn comes out even from fence to foundation, with a sharp line where the sidewalk starts. Every week, that same even result is the whole reason to show up.