YES

   1: +(0(),x1) -> x1
   2: +(s(x2),x1) -> s(+(x2,x1))
   3: +(x2,0()) -> x2
   4: +(x2,s(x1)) -> s(+(x2,x1))

@Development Closedness
--- R
   1: +(0(),x1) -> x1
   2: +(s(x2),x1) -> s(+(x2,x1))
   3: +(x2,0()) -> x2
   4: +(x2,s(x1)) -> s(+(x2,x1))

--- S
   1: +(0(),x1) -> x1
   2: +(s(x2),x1) -> s(+(x2,x1))
   3: +(x2,0()) -> x2
   4: +(x2,s(x1)) -> s(+(x2,x1))