YES

1 decompositions
 #1 -----------
   1: a() -> b()
   2: a() -> f(a())
   3: f(x1) -> x1

@Development Closedness
--- R
   1: a() -> b()
   2: a() -> f(a())
   3: f(x1) -> x1

--- S
   1: a() -> b()
   2: a() -> f(a())
   3: f(x1) -> x1