YES
LCTRS
  Theories
    Core, Ints
  Sorts
    Unit
  Signature
    fib: Int -> Unit
    return: Int -> Unit
    w: (Unit, Unit) -> Unit
  Rules
    fib(?18) -> return(0) [<=(?18, 0)]
    fib(?19) -> return(1) [?19 = 1]
    fib(?20) -> w(fib(-(?20, 1)), fib(-(?20, 2))) [>=(-(?20, 2), 0)]
    w(return(?21), return(?22)) -> return(+(?21, ?22))
Confluent by Weak Orthogonality with proof:
  no critical pairs

Elapsed Time:  42.18 ms