YES

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

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

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