YES

# Compositional parallel critical pair system (Shintani and Hirokawa 2022).

Consider the left-linear TRS R:

  c() -> b()
  a() -> a()
  b() -> b()
  f(f(a())) -> c()

Let C be the following subset of R:

  c() -> b()
  a() -> a()
  b() -> b()
  f(f(a())) -> c()

The parallel critical pair system PCPS(R,C) is:

  (empty)

All pairs in PCP(R) are joinable and PCPS(R,C)/R is terminating.
Therefore, the confluence of R follows from that of C.

# Parallel rule labeling (Zankl et al. 2015).

Consider the left-linear TRS R:

  c() -> b()
  a() -> a()
  b() -> b()
  f(f(a())) -> c()

All parallel critical peaks (except C's) are decreasing wrt rule labeling:

  phi(c() -> b()) = 1
  phi(a() -> a()) = 1
  phi(b() -> b()) = 1
  phi(f(f(a())) -> c()) = 1

  psi(c() -> b()) = 1
  psi(a() -> a()) = 1
  psi(b() -> b()) = 1
  psi(f(f(a())) -> c()) = 1