YES

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

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

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