NO

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

unjoinable peak
    b()
*<- f(b(),a()) ->*
    a()
NOTE: input TRS is reduced

original is
   1: a() -> a()
   2: f(b(),x1) -> b()
   3: f(x2,a()) -> a()

reduced to
   2: f(b(),x1) -> b()
   3: f(x2,a()) -> a()