code: function(vec2, returnNew) {
        if (!returnNew) {
          this.x += vec2.x; this.y += vec2.y;
          return this.change();
        } else {
          // Return a new vector if `returnNew` is truthy
          return new Vec2(
            this.x + vec2.x,
            this.y + vec2.y
          );
        }
      }
docs: Add the incoming `vec2` vector to this vector