YES

1 decompositions
 #1 -----------
   1: f(a(),x1) -> f(a(),g(x1))
   2: a() -> b()
   3: g(x2) -> x2

@Rule Labeling
--- R
   1: f(a(),x1) -> f(a(),g(x1))
   2: a() -> b()
   3: g(x2) -> x2

--- S
   1: f(a(),x1) -> f(a(),g(x1))
   2: a() -> b()
   3: g(x2) -> x2