NO

   1: f(g(x1,a(),b())) -> x1
   2: p(a()) -> c()
   3: g(f(h(c(),d())),x1,x2) -> h(p(x1),q(x1))
   4: q(b()) -> d()

unjoinable pair
  f(h(c(),q(a()))) <- -> f(h(c(),d()))