YES

1 decompositions
 #0 -----------
   1: f(a(),a()) -> b()
   2: a() -> a'()
   3: f(a'(),x) -> f(x,x)
   4: f(x,a'()) -> f(x,x)
   5: f(a'(),a'()) -> b()
   6: b() -> f(a'(),a'())

@Parallel Closedness
--- R
   1: f(a(),a()) -> b()
   2: a() -> a'()
   3: f(a'(),x) -> f(x,x)
   4: f(x,a'()) -> f(x,x)
   5: f(a'(),a'()) -> b()
   6: b() -> f(a'(),a'())

--- S
   1: f(a(),a()) -> b()
   2: a() -> a'()
   3: f(a'(),x) -> f(x,x)
   4: f(x,a'()) -> f(x,x)
   5: f(a'(),a'()) -> b()
   6: b() -> f(a'(),a'())